diff --git a/lib/legacy/class.html b/lib/legacy/class.html
index 7a7436d8f3..7411d4bb15 100644
--- a/lib/legacy/class.html
+++ b/lib/legacy/class.html
@@ -126,7 +126,7 @@
             }
           }
         } else {
-          Polymer._warn('behavior is null, check for missing or 404 import');
+          console.warn('behavior is null, check for missing or 404 import');
         }
       }
       return list;
@@ -312,7 +312,7 @@
      */
     Polymer.Class = function(info) {
       if (!info) {
-        Polymer._warn('Polymer.Class requires `info` argument');
+        console.warn('Polymer.Class requires `info` argument');
       }
       let klass = GenerateClassFromInfo(info, info.behaviors ?
         // note: mixinBehaviors ensures `LegacyElementMixin`.
diff --git a/lib/legacy/templatizer-behavior.html b/lib/legacy/templatizer-behavior.html
index 968db24197..653e52e5f8 100644
--- a/lib/legacy/templatizer-behavior.html
+++ b/lib/legacy/templatizer-behavior.html
@@ -84,11 +84,15 @@
        * by `stamp` to create new instances of the template.
        *
        * @param {HTMLTemplateElement} template Template to prepare
+       * @param {Boolean=} mutableData When `true`, the generated class will skip
+       *   strict dirty-checking for objects and arrays (always consider them to
+       *   be "dirty"). Defaults to false.
        * @return {TemplateInstance} Description
        */
-      templatize(template) {
+      templatize(template, mutableData) {
         this._templatizerTemplate = template;
         this.ctor = Polymer.Templatize.templatize(template, this, {
+          mutableData: Boolean(mutableData),
           parentModel: this._parentModel,
           instanceProps: this._instanceProps,
           forwardHostProp: this._forwardHostPropV2,
diff --git a/lib/utils/templatize.html b/lib/utils/templatize.html
index a4bf1335fb..ad1008aee3 100644
--- a/lib/utils/templatize.html
+++ b/lib/utils/templatize.html
@@ -354,6 +354,9 @@
        *   to the instance by the templatize owner.  These properties shadow any
        *   host properties, and changes within the template to these properties
        *   will result in `notifyInstanceProperties` to be called.
+       * - `mutableData`: When `true`, the generated class will skip strict
+       *   dirty-checking for objects and arrays (always consider them to be
+       *   "dirty").
        * - `notifyInstanceProperties(instance, property, value)`: Called when
        *   an instance property changes.  Users may choose to call `notifyPath`
        *   on e.g. the owner to notify the change.
diff --git a/polymer.html b/polymer.html
index 130a5c9ef7..0331373cd6 100644
--- a/polymer.html
+++ b/polymer.html
@@ -17,6 +17,8 @@
 <link rel="import" href="lib/elements/array-selector.html">
 <!-- custom-style -->
 <link rel="import" href="lib/elements/custom-style.html">
+<!-- bc behaviors -->
+<link rel="import" href="lib/legacy/mutable-data-behavior.html">
 <script>
   // bc
   Polymer.Base = Polymer.LegacyElementMixin(HTMLElement).prototype;
diff --git a/test/unit/behaviors.html b/test/unit/behaviors.html
index daafb592cb..27535891e3 100644
--- a/test/unit/behaviors.html
+++ b/test/unit/behaviors.html
@@ -414,11 +414,7 @@
   });
 
   test('behavior is null generates warning', function() {
-    var warned = false, oldWarn = Polymer._warn;
-    Polymer._warn = function(message) {
-      assert.match(message, /behavior is null/);
-      warned = true;
-    };
+    sinon.spy(console, 'warn');
     Polymer({
       is: 'behavior-null',
       behaviors: [
@@ -426,8 +422,8 @@
       ]
     });
     document.createElement('behavior-null');
-    assert.equal(warned, true, 'Null behaviour should generate warning');
-    Polymer.Base._warn = oldWarn;
+    assert.equal(console.warn.callCount, 1, 'Null behaviour should generate warning');
+    console.warn.restore();
   });
 
   test('behavior array is unique', function() {
diff --git a/test/unit/mixin-behaviors.html b/test/unit/mixin-behaviors.html
index 139b83b870..3a598b6318 100644
--- a/test/unit/mixin-behaviors.html
+++ b/test/unit/mixin-behaviors.html
@@ -495,11 +495,7 @@
   });
 
   test('behavior is null generates warning', function() {
-    var warned = false, oldWarn = Polymer._warn;
-    Polymer._warn = function(message) {
-      assert.match(message, /behavior is null/);
-      warned = true;
-    };
+    sinon.spy(console, 'warn');
     Polymer({
       is: 'behavior-null',
       behaviors: [
@@ -507,8 +503,8 @@
       ]
     });
     document.createElement('behavior-null');
-    assert.equal(warned, true, 'Null behaviour should generate warning');
-    Polymer.Base._warn = oldWarn;
+    assert.equal(console.warn.callCount, 1, 'Null behaviour should generate warning');
+    console.warn.restore();
   });
 
   test('behavior array is unique', function() {
diff --git a/test/unit/property-effects-elements.html b/test/unit/property-effects-elements.html
index 4e4e1344ce..7005ec55b3 100644
--- a/test/unit/property-effects-elements.html
+++ b/test/unit/property-effects-elements.html
@@ -7,9 +7,6 @@
 Code distributed by Google as part of the polymer project is also
 subject to an additional IP rights grant found at http://polymer.github.io/PATENTS.txt
 -->
-<link rel="import" href="../../lib/legacy/mutable-data-behavior.html">
-<link rel="import" href="../../lib/mixins/mutable-data.html">
-
 <dom-module id="x-basic">
   <template>
     <div id="boundChild"