diff --git a/lib/mixins/element-mixin.js b/lib/mixins/element-mixin.js
index 84c590686a..6c0f0692da 100644
--- a/lib/mixins/element-mixin.js
+++ b/lib/mixins/element-mixin.js
@@ -195,7 +195,6 @@ export const ElementMixin = dedupingMixin(base => {
    * disables the effect, the setter would fail unexpectedly.
    * Based on feedback, we may want to try to make effects more malleable
    * and/or provide an advanced api for manipulating them.
-   * Also consider adding warnings when an effect cannot be changed.
    *
    * @param {!PolymerElement} proto Element class prototype to add accessors
    *   and effects to
@@ -217,17 +216,27 @@ export const ElementMixin = dedupingMixin(base => {
     // setup where multiple triggers for setting a property)
     // While we do have `hasComputedEffect` this is set on the property's
     // dependencies rather than itself.
-    if (info.computed && !proto._hasReadOnlyEffect(name)) {
-      proto._createComputedProperty(name, info.computed, allProps);
+    if (info.computed) {
+      if (proto._hasReadOnlyEffect(name)) {
+        console.warn(`Cannot redefine computed property '${name}'.`);
+      } else {
+        proto._createComputedProperty(name, info.computed, allProps);
+      }
     }
     if (info.readOnly && !proto._hasReadOnlyEffect(name)) {
       proto._createReadOnlyProperty(name, !info.computed);
+    } else if (info.readOnly === false && proto._hasReadOnlyEffect(name)) {
+      console.warn(`Cannot make readOnly property '${name}' non-readOnly.`);
     }
     if (info.reflectToAttribute && !proto._hasReflectEffect(name)) {
       proto._createReflectedProperty(name);
+    } else if (info.reflectToAttribute === false && proto._hasReflectEffect(name)) {
+      console.warn(`Cannot make reflected property '${name}' non-reflected.`);
     }
     if (info.notify && !proto._hasNotifyEffect(name)) {
       proto._createNotifyingProperty(name);
+    } else if (info.notify === false && proto._hasNotifyEffect(name)) {
+      console.warn(`Cannot make notify property '${name}' non-notify.`);
     }
     // always add observer
     if (info.observer) {
@@ -718,7 +727,7 @@ export const ElementMixin = dedupingMixin(base => {
     }
 
     /**
-     * Overrides `PropertyAccessors` to add map of dynamic functions on
+     * Overrides `PropertyEffects` to add map of dynamic functions on
      * template info, for consumption by `PropertyEffects` template binding
      * code. This map determines which method templates should have accessors
      * created for them.
@@ -734,6 +743,32 @@ export const ElementMixin = dedupingMixin(base => {
       return super._parseTemplateContent(template, templateInfo, nodeInfo);
     }
 
+    /**
+     * Overrides `PropertyEffects` to warn on use of undeclared properties in
+     * template.
+     *
+     * @param {Object} templateInfo Template metadata to add effect to
+     * @param {string} prop Property that should trigger the effect
+     * @param {Object=} effect Effect metadata object
+     * @return {void}
+     * @protected
+     */
+    static _addTemplatePropertyEffect(templateInfo, prop, effect) {
+      // Warn if properties are used in template without being declared.
+      // Properties must be listed in `properties` to be included in
+      // `observedAttributes` since CE V1 reads that at registration time, and
+      // since we want to keep template parsing lazy, we can't automatically
+      // add undeclared properties used in templates to `observedAttributes`.
+      // The warning is only enabled in `legacyOptimizations` mode, since
+      // we don't want to spam existing users who might have adopted the
+      // shorthand when attribute deserialization is not important.
+      if (legacyOptimizations && !(prop in this._properties)) {
+        console.warn(`Property '${prop}' used in template but not declared in 'properties'; ` +
+          `attribute will not be observed.`);
+      }
+      return super._addTemplatePropertyEffect(templateInfo, prop, effect);
+    }
+
   }
 
   return PolymerElement;
diff --git a/package-lock.json b/package-lock.json
index 9da912d1cb..d3fd24d2f2 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1310,9 +1310,9 @@
       "integrity": "sha512-bx0TzeZ11VqYDGLuXfznen8+4u0hADk2dD5RNMFxWL9MM4c5NXCDCgNXgssb4i2zQOos/GGe4tl5AuE0LzJkLA=="
     },
     "@webcomponents/webcomponentsjs": {
-      "version": "2.2.2",
-      "resolved": "https://registry.npmjs.org/@webcomponents/webcomponentsjs/-/webcomponentsjs-2.2.2.tgz",
-      "integrity": "sha512-t45WI6m/dD+gmMaQ/u0k1gCrSg5yESnfGaxfFPV1GST2RL/aEScoaotFsfjfP8DPcq2OH6leyHoWBT+pSuGsdQ==",
+      "version": "2.2.6",
+      "resolved": "https://registry.npmjs.org/@webcomponents/webcomponentsjs/-/webcomponentsjs-2.2.6.tgz",
+      "integrity": "sha512-TbuyV7hHIq4jMlvQ8pF3C6QNEqFeBX4be9AND5DO8UCRRYcZBpjl1yH9IChbebeWr1QNcK5WlZC3voAlQCdisQ==",
       "dev": true
     },
     "abbrev": {
@@ -9756,7 +9756,7 @@
         },
         "array-flatten": {
           "version": "1.1.1",
-          "resolved": "http://registry.npmjs.org/array-flatten/-/array-flatten-1.1.1.tgz",
+          "resolved": "https://registry.npmjs.org/array-flatten/-/array-flatten-1.1.1.tgz",
           "integrity": "sha1-ml9pkFGx5wczKPKgCJaLZOopVdI=",
           "dev": true
         },
@@ -10635,7 +10635,7 @@
             },
             "string_decoder": {
               "version": "0.10.31",
-              "resolved": "http://registry.npmjs.org/string_decoder/-/string_decoder-0.10.31.tgz",
+              "resolved": "https://registry.npmjs.org/string_decoder/-/string_decoder-0.10.31.tgz",
               "integrity": "sha1-YuIDvEF2bGwoyfyEMB2rHFMQ+pQ=",
               "dev": true
             }
@@ -11119,7 +11119,7 @@
         },
         "concat-stream": {
           "version": "1.6.2",
-          "resolved": "http://registry.npmjs.org/concat-stream/-/concat-stream-1.6.2.tgz",
+          "resolved": "https://registry.npmjs.org/concat-stream/-/concat-stream-1.6.2.tgz",
           "integrity": "sha512-27HBghJxjiZtIk3Ycvn/4kbJk/1uZuJFfuPEns6LaEvpvG1f0hTea8lilrouyo9mVc2GWdcEZ8OLoGmSADlrCw==",
           "dev": true,
           "requires": {
@@ -11145,7 +11145,7 @@
         },
         "content-disposition": {
           "version": "0.5.2",
-          "resolved": "http://registry.npmjs.org/content-disposition/-/content-disposition-0.5.2.tgz",
+          "resolved": "https://registry.npmjs.org/content-disposition/-/content-disposition-0.5.2.tgz",
           "integrity": "sha1-DPaLud318r55YcOoUXjLhdunjLQ=",
           "dev": true
         },
@@ -11529,7 +11529,7 @@
             },
             "string_decoder": {
               "version": "0.10.31",
-              "resolved": "http://registry.npmjs.org/string_decoder/-/string_decoder-0.10.31.tgz",
+              "resolved": "https://registry.npmjs.org/string_decoder/-/string_decoder-0.10.31.tgz",
               "integrity": "sha1-YuIDvEF2bGwoyfyEMB2rHFMQ+pQ=",
               "dev": true
             }
@@ -11895,7 +11895,7 @@
         },
         "expand-range": {
           "version": "1.8.2",
-          "resolved": "http://registry.npmjs.org/expand-range/-/expand-range-1.8.2.tgz",
+          "resolved": "https://registry.npmjs.org/expand-range/-/expand-range-1.8.2.tgz",
           "integrity": "sha1-opnv/TNf4nIeuujiV+x5ZE/IUzc=",
           "dev": true,
           "requires": {
@@ -12144,7 +12144,7 @@
                 },
                 "is-accessor-descriptor": {
                   "version": "0.1.6",
-                  "resolved": "http://registry.npmjs.org/is-accessor-descriptor/-/is-accessor-descriptor-0.1.6.tgz",
+                  "resolved": "https://registry.npmjs.org/is-accessor-descriptor/-/is-accessor-descriptor-0.1.6.tgz",
                   "integrity": "sha1-qeEss66Nh2cn7u84Q/igiXtcmNY=",
                   "dev": true,
                   "requires": {
@@ -12164,7 +12164,7 @@
                 },
                 "is-data-descriptor": {
                   "version": "0.1.4",
-                  "resolved": "http://registry.npmjs.org/is-data-descriptor/-/is-data-descriptor-0.1.4.tgz",
+                  "resolved": "https://registry.npmjs.org/is-data-descriptor/-/is-data-descriptor-0.1.4.tgz",
                   "integrity": "sha1-C17mSDiOLIYCgueT8YVv7D8wG1Y=",
                   "dev": true,
                   "requires": {
@@ -12683,15 +12683,13 @@
               "version": "1.0.0",
               "resolved": "https://registry.npmjs.org/balanced-match/-/balanced-match-1.0.0.tgz",
               "integrity": "sha1-ibTRmasr7kneFk6gK4nORi1xt2c=",
-              "dev": true,
-              "optional": true
+              "dev": true
             },
             "brace-expansion": {
               "version": "1.1.11",
               "resolved": "https://registry.npmjs.org/brace-expansion/-/brace-expansion-1.1.11.tgz",
               "integrity": "sha512-iCuPHDFgrHX7H2vEI/5xpz07zSHB00TpugqhmYtVmMO6518mCuRMoOYFldEBl0g187ufozdaHgWKcYFb61qGiA==",
               "dev": true,
-              "optional": true,
               "requires": {
                 "balanced-match": "^1.0.0",
                 "concat-map": "0.0.1"
@@ -12708,22 +12706,19 @@
               "version": "1.1.0",
               "resolved": "https://registry.npmjs.org/code-point-at/-/code-point-at-1.1.0.tgz",
               "integrity": "sha1-DQcLTQQ6W+ozovGkDi7bPZpMz3c=",
-              "dev": true,
-              "optional": true
+              "dev": true
             },
             "concat-map": {
               "version": "0.0.1",
               "resolved": "https://registry.npmjs.org/concat-map/-/concat-map-0.0.1.tgz",
               "integrity": "sha1-2Klr13/Wjfd5OnMDajug1UBdR3s=",
-              "dev": true,
-              "optional": true
+              "dev": true
             },
             "console-control-strings": {
               "version": "1.1.0",
               "resolved": "https://registry.npmjs.org/console-control-strings/-/console-control-strings-1.1.0.tgz",
               "integrity": "sha1-PXz0Rk22RG6mRL9LOVB/mFEAjo4=",
-              "dev": true,
-              "optional": true
+              "dev": true
             },
             "core-util-is": {
               "version": "1.0.2",
@@ -12854,8 +12849,7 @@
               "version": "2.0.3",
               "resolved": "https://registry.npmjs.org/inherits/-/inherits-2.0.3.tgz",
               "integrity": "sha1-Yzwsg+PaQqUC9SRmAiSA9CCCYd4=",
-              "dev": true,
-              "optional": true
+              "dev": true
             },
             "ini": {
               "version": "1.3.5",
@@ -12869,7 +12863,6 @@
               "resolved": "https://registry.npmjs.org/is-fullwidth-code-point/-/is-fullwidth-code-point-1.0.0.tgz",
               "integrity": "sha1-754xOG8DGn8NZDr4L95QxFfvAMs=",
               "dev": true,
-              "optional": true,
               "requires": {
                 "number-is-nan": "^1.0.0"
               }
@@ -12886,7 +12879,6 @@
               "resolved": "https://registry.npmjs.org/minimatch/-/minimatch-3.0.4.tgz",
               "integrity": "sha512-yJHVQEhyqPLUTgt9B83PXu6W3rx4MvvHvSUvToogpwoGDOUQ+yDrR0HRot+yOCdCO7u4hX3pWft6kWBBcqh0UA==",
               "dev": true,
-              "optional": true,
               "requires": {
                 "brace-expansion": "^1.1.7"
               }
@@ -12895,15 +12887,13 @@
               "version": "0.0.8",
               "resolved": "https://registry.npmjs.org/minimist/-/minimist-0.0.8.tgz",
               "integrity": "sha1-hX/Kv8M5fSYluCKCYuhqp6ARsF0=",
-              "dev": true,
-              "optional": true
+              "dev": true
             },
             "minipass": {
               "version": "2.2.4",
               "resolved": "https://registry.npmjs.org/minipass/-/minipass-2.2.4.tgz",
               "integrity": "sha512-hzXIWWet/BzWhYs2b+u7dRHlruXhwdgvlTMDKC6Cb1U7ps6Ac6yQlR39xsbjWJE377YTCtKwIXIpJ5oP+j5y8g==",
               "dev": true,
-              "optional": true,
               "requires": {
                 "safe-buffer": "^5.1.1",
                 "yallist": "^3.0.0"
@@ -12924,7 +12914,6 @@
               "resolved": "https://registry.npmjs.org/mkdirp/-/mkdirp-0.5.1.tgz",
               "integrity": "sha1-MAV0OOrGz3+MR2fzhkjWaX11yQM=",
               "dev": true,
-              "optional": true,
               "requires": {
                 "minimist": "0.0.8"
               }
@@ -13013,8 +13002,7 @@
               "version": "1.0.1",
               "resolved": "https://registry.npmjs.org/number-is-nan/-/number-is-nan-1.0.1.tgz",
               "integrity": "sha1-CXtgK1NCKlIsGvuHkDGDNpQaAR0=",
-              "dev": true,
-              "optional": true
+              "dev": true
             },
             "object-assign": {
               "version": "4.1.1",
@@ -13028,7 +13016,6 @@
               "resolved": "https://registry.npmjs.org/once/-/once-1.4.0.tgz",
               "integrity": "sha1-WDsap3WWHUsROsF9nFC6753Xa9E=",
               "dev": true,
-              "optional": true,
               "requires": {
                 "wrappy": "1"
               }
@@ -13166,7 +13153,6 @@
               "resolved": "https://registry.npmjs.org/string-width/-/string-width-1.0.2.tgz",
               "integrity": "sha1-EYvfW4zcUaKn5w0hHgfisLmxB9M=",
               "dev": true,
-              "optional": true,
               "requires": {
                 "code-point-at": "^1.0.0",
                 "is-fullwidth-code-point": "^1.0.0",
@@ -13423,7 +13409,7 @@
             },
             "string_decoder": {
               "version": "0.10.31",
-              "resolved": "http://registry.npmjs.org/string_decoder/-/string_decoder-0.10.31.tgz",
+              "resolved": "https://registry.npmjs.org/string_decoder/-/string_decoder-0.10.31.tgz",
               "integrity": "sha1-YuIDvEF2bGwoyfyEMB2rHFMQ+pQ=",
               "dev": true
             },
@@ -13456,7 +13442,7 @@
         },
         "global-modules": {
           "version": "0.2.3",
-          "resolved": "http://registry.npmjs.org/global-modules/-/global-modules-0.2.3.tgz",
+          "resolved": "https://registry.npmjs.org/global-modules/-/global-modules-0.2.3.tgz",
           "integrity": "sha1-6lo77ULG1s6ZWk+KEmm12uIjgo0=",
           "dev": true,
           "requires": {
@@ -13474,7 +13460,7 @@
         },
         "global-prefix": {
           "version": "0.1.5",
-          "resolved": "http://registry.npmjs.org/global-prefix/-/global-prefix-0.1.5.tgz",
+          "resolved": "https://registry.npmjs.org/global-prefix/-/global-prefix-0.1.5.tgz",
           "integrity": "sha1-jTvGuNo8qBEqFg2NSW/wRiv+948=",
           "dev": true,
           "requires": {
@@ -14009,7 +13995,7 @@
         },
         "is-accessor-descriptor": {
           "version": "0.1.6",
-          "resolved": "http://registry.npmjs.org/is-accessor-descriptor/-/is-accessor-descriptor-0.1.6.tgz",
+          "resolved": "https://registry.npmjs.org/is-accessor-descriptor/-/is-accessor-descriptor-0.1.6.tgz",
           "integrity": "sha1-qeEss66Nh2cn7u84Q/igiXtcmNY=",
           "dev": true,
           "requires": {
@@ -14057,7 +14043,7 @@
         },
         "is-data-descriptor": {
           "version": "0.1.4",
-          "resolved": "http://registry.npmjs.org/is-data-descriptor/-/is-data-descriptor-0.1.4.tgz",
+          "resolved": "https://registry.npmjs.org/is-data-descriptor/-/is-data-descriptor-0.1.4.tgz",
           "integrity": "sha1-C17mSDiOLIYCgueT8YVv7D8wG1Y=",
           "dev": true,
           "requires": {
@@ -15046,7 +15032,7 @@
         },
         "multimatch": {
           "version": "2.1.0",
-          "resolved": "http://registry.npmjs.org/multimatch/-/multimatch-2.1.0.tgz",
+          "resolved": "https://registry.npmjs.org/multimatch/-/multimatch-2.1.0.tgz",
           "integrity": "sha1-nHkGoi+0wCkZ4vX3UWG0zb1LKis=",
           "dev": true,
           "requires": {
@@ -15058,7 +15044,7 @@
         },
         "multipipe": {
           "version": "1.0.2",
-          "resolved": "http://registry.npmjs.org/multipipe/-/multipipe-1.0.2.tgz",
+          "resolved": "https://registry.npmjs.org/multipipe/-/multipipe-1.0.2.tgz",
           "integrity": "sha1-zBPv2DPJzamfIk+GhGG44aP9k50=",
           "dev": true,
           "requires": {
@@ -15495,7 +15481,7 @@
         },
         "pako": {
           "version": "0.2.9",
-          "resolved": "http://registry.npmjs.org/pako/-/pako-0.2.9.tgz",
+          "resolved": "https://registry.npmjs.org/pako/-/pako-0.2.9.tgz",
           "integrity": "sha1-8/dSL073gjSNqBYbrZ7P1Rv4OnU=",
           "dev": true
         },
@@ -16079,7 +16065,7 @@
         },
         "pretty-bytes": {
           "version": "4.0.2",
-          "resolved": "http://registry.npmjs.org/pretty-bytes/-/pretty-bytes-4.0.2.tgz",
+          "resolved": "https://registry.npmjs.org/pretty-bytes/-/pretty-bytes-4.0.2.tgz",
           "integrity": "sha1-sr+C5zUNZcbDOqlaqlpPYyf2HNk=",
           "dev": true
         },
@@ -16378,7 +16364,7 @@
                 },
                 "is-accessor-descriptor": {
                   "version": "0.1.6",
-                  "resolved": "http://registry.npmjs.org/is-accessor-descriptor/-/is-accessor-descriptor-0.1.6.tgz",
+                  "resolved": "https://registry.npmjs.org/is-accessor-descriptor/-/is-accessor-descriptor-0.1.6.tgz",
                   "integrity": "sha1-qeEss66Nh2cn7u84Q/igiXtcmNY=",
                   "dev": true,
                   "requires": {
@@ -16398,7 +16384,7 @@
                 },
                 "is-data-descriptor": {
                   "version": "0.1.4",
-                  "resolved": "http://registry.npmjs.org/is-data-descriptor/-/is-data-descriptor-0.1.4.tgz",
+                  "resolved": "https://registry.npmjs.org/is-data-descriptor/-/is-data-descriptor-0.1.4.tgz",
                   "integrity": "sha1-C17mSDiOLIYCgueT8YVv7D8wG1Y=",
                   "dev": true,
                   "requires": {
@@ -16792,7 +16778,7 @@
         },
         "resolve-dir": {
           "version": "0.1.1",
-          "resolved": "http://registry.npmjs.org/resolve-dir/-/resolve-dir-0.1.1.tgz",
+          "resolved": "https://registry.npmjs.org/resolve-dir/-/resolve-dir-0.1.1.tgz",
           "integrity": "sha1-shklmlYC+sXFxJatiUpujMQwJh4=",
           "dev": true,
           "requires": {
@@ -17555,7 +17541,7 @@
         },
         "string-width": {
           "version": "1.0.2",
-          "resolved": "http://registry.npmjs.org/string-width/-/string-width-1.0.2.tgz",
+          "resolved": "https://registry.npmjs.org/string-width/-/string-width-1.0.2.tgz",
           "integrity": "sha1-EYvfW4zcUaKn5w0hHgfisLmxB9M=",
           "dev": true,
           "requires": {
@@ -17566,7 +17552,7 @@
         },
         "string_decoder": {
           "version": "1.1.1",
-          "resolved": "http://registry.npmjs.org/string_decoder/-/string_decoder-1.1.1.tgz",
+          "resolved": "https://registry.npmjs.org/string_decoder/-/string_decoder-1.1.1.tgz",
           "integrity": "sha512-n/ShnvDi6FHbbVfviro+WojiFzv+s8MPMHBczVePfUpDJLwoLT0ht1l4YwBCbi8pJAveEEdnkHyPyTP/mzRfwg==",
           "dev": true,
           "requires": {
@@ -17624,7 +17610,7 @@
         },
         "supports-color": {
           "version": "2.0.0",
-          "resolved": "http://registry.npmjs.org/supports-color/-/supports-color-2.0.0.tgz",
+          "resolved": "https://registry.npmjs.org/supports-color/-/supports-color-2.0.0.tgz",
           "integrity": "sha1-U10EXOa2Nj+kARcIRimZXp3zJMc=",
           "dev": true
         },
@@ -17767,7 +17753,7 @@
         },
         "temp": {
           "version": "0.8.3",
-          "resolved": "http://registry.npmjs.org/temp/-/temp-0.8.3.tgz",
+          "resolved": "https://registry.npmjs.org/temp/-/temp-0.8.3.tgz",
           "integrity": "sha1-4Ma8TSa5AxJEEOT+2BEDAU38H1k=",
           "dev": true,
           "requires": {
@@ -18116,7 +18102,7 @@
         },
         "underscore": {
           "version": "1.6.0",
-          "resolved": "http://registry.npmjs.org/underscore/-/underscore-1.6.0.tgz",
+          "resolved": "https://registry.npmjs.org/underscore/-/underscore-1.6.0.tgz",
           "integrity": "sha1-izixDKze9jM3uLJOT/htRa6lKag=",
           "dev": true
         },
@@ -18302,7 +18288,7 @@
           "dependencies": {
             "ansi-align": {
               "version": "1.1.0",
-              "resolved": "http://registry.npmjs.org/ansi-align/-/ansi-align-1.1.0.tgz",
+              "resolved": "https://registry.npmjs.org/ansi-align/-/ansi-align-1.1.0.tgz",
               "integrity": "sha1-LwwWWIKXOa3V67FeawxuNCPwFro=",
               "dev": true,
               "requires": {
@@ -18930,7 +18916,7 @@
                 },
                 "is-accessor-descriptor": {
                   "version": "0.1.6",
-                  "resolved": "http://registry.npmjs.org/is-accessor-descriptor/-/is-accessor-descriptor-0.1.6.tgz",
+                  "resolved": "https://registry.npmjs.org/is-accessor-descriptor/-/is-accessor-descriptor-0.1.6.tgz",
                   "integrity": "sha1-qeEss66Nh2cn7u84Q/igiXtcmNY=",
                   "dev": true,
                   "requires": {
@@ -18950,7 +18936,7 @@
                 },
                 "is-data-descriptor": {
                   "version": "0.1.4",
-                  "resolved": "http://registry.npmjs.org/is-data-descriptor/-/is-data-descriptor-0.1.4.tgz",
+                  "resolved": "https://registry.npmjs.org/is-data-descriptor/-/is-data-descriptor-0.1.4.tgz",
                   "integrity": "sha1-C17mSDiOLIYCgueT8YVv7D8wG1Y=",
                   "dev": true,
                   "requires": {
@@ -21021,7 +21007,7 @@
     },
     "stream-combiner": {
       "version": "0.2.2",
-      "resolved": "https://registry.npmjs.org/stream-combiner/-/stream-combiner-0.2.2.tgz",
+      "resolved": "http://registry.npmjs.org/stream-combiner/-/stream-combiner-0.2.2.tgz",
       "integrity": "sha1-rsjLrBd7Vrb0+kec7YwZEs7lKFg=",
       "dev": true,
       "requires": {
diff --git a/package.json b/package.json
index 26c688e629..3fcebe360a 100644
--- a/package.json
+++ b/package.json
@@ -12,7 +12,7 @@
     "@polymer/gen-typescript-declarations": "^1.5.1",
     "@polymer/iron-component-page": "^3.0.0-pre.12",
     "@polymer/test-fixture": "^3.0.0-pre.12",
-    "@webcomponents/webcomponentsjs": "^2.2.2",
+    "@webcomponents/webcomponentsjs": "^2.2.6",
     "babel-eslint": "^7.2.3",
     "babel-preset-minify": "^0.2.0",
     "del": "^3.0.0",
diff --git a/test/unit/property-effects.html b/test/unit/property-effects.html
index fbafbed8a9..087023914f 100644
--- a/test/unit/property-effects.html
+++ b/test/unit/property-effects.html
@@ -25,8 +25,8 @@
 
 <script type="module">
 import './property-effects-elements.js';
-import { Polymer } from '../../lib/legacy/polymer-fn.js';
-import { setSanitizeDOMValue, sanitizeDOMValue } from '../../lib/utils/settings.js';
+import { Polymer, html } from '../../polymer-legacy.js';
+import { setSanitizeDOMValue, sanitizeDOMValue, setLegacyOptimizations } from '../../lib/utils/settings.js';
 import { PropertyEffects } from '../../lib/mixins/property-effects.js';
 
 suite('single-element binding effects', function() {
@@ -1872,6 +1872,110 @@
     document.body.removeChild(el);
   });
 
+});
+
+suite('warn on legacy differences', () => {
+
+  setup(function() {
+    sinon.spy(console, 'warn');
+  });
+
+  teardown(function() {
+    setLegacyOptimizations(false);
+    console.warn.restore();
+  });
+
+  test('warn if non-declared property used in binding', () => {
+    setLegacyOptimizations(true);
+    Polymer({
+      is: 'x-warn-undeclared-binding',
+      _template: html`
+        <div attr$="[[undeclaredToAttr]]"
+             prop="[[undeclaredToProp]]">
+             [[undeclaredToText]]</div>`
+    });
+    const el = document.createElement('x-warn-undeclared-binding');
+    document.body.appendChild(el);
+    document.body.removeChild(el);
+    assert.equal(console.warn.callCount, 3);
+  });
+
+  test('warn when re-declaring a computed property', () => {
+    Polymer({
+      is: 'x-warn-redeclared-computed',
+      behaviors: [{
+        properties: {
+          a: { computed: 'compute(x)' }
+        }
+      }, {
+        properties: {
+          a: { computed: 'compute(y)' }
+        }
+      }]
+    });
+    const el = document.createElement('x-warn-redeclared-computed');
+    document.body.appendChild(el);
+    document.body.removeChild(el);
+    assert.equal(console.warn.callCount, 1);
+  });
+
+  test('warn when disabling readOnly on a readOnly property', () => {
+    Polymer({
+      is: 'x-warn-disable-readonly',
+      behaviors: [{
+        properties: {
+          a: { readOnly: true }
+        }
+      }, {
+        properties: {
+          a: { readOnly: false }
+        }
+      }]
+    });
+    const el = document.createElement('x-warn-disable-readonly');
+    document.body.appendChild(el);
+    document.body.removeChild(el);
+    assert.equal(console.warn.callCount, 1);
+  });
+
+  test('warn when disabling reflect on a reflect property', () => {
+    Polymer({
+      is: 'x-warn-disable-reflect',
+      behaviors: [{
+        properties: {
+          a: { reflectToAttribute: true }
+        }
+      }, {
+        properties: {
+          a: { reflectToAttribute: false }
+        }
+      }]
+    });
+    const el = document.createElement('x-warn-disable-reflect');
+    document.body.appendChild(el);
+    document.body.removeChild(el);
+    assert.equal(console.warn.callCount, 1);
+  });
+
+  test('warn when disabling notify on a notify property', () => {
+    Polymer({
+      is: 'x-warn-disable-notify',
+      behaviors: [{
+        properties: {
+          a: { notify: true }
+        }
+      }, {
+        properties: {
+          a: { notify: false }
+        }
+      }]
+    });
+    const el = document.createElement('x-warn-disable-notify');
+    document.body.appendChild(el);
+    document.body.removeChild(el);
+    assert.equal(console.warn.callCount, 1);
+  });
+
 });
 </script>