diff --git a/lib/legacy/class.html b/lib/legacy/class.html
index e416c58314..ce3ff47214 100644
--- a/lib/legacy/class.html
+++ b/lib/legacy/class.html
@@ -155,58 +155,6 @@
       return list;
     }
 
-    function mergePropertyInfo(a, b) {
-      if ('value' in b) {
-        a.value = b.value;
-      // ensure property value is always overridden.
-      } else if ('value' in a) {
-        delete a.value;
-      }
-      if (typeof b === 'function') {
-        a.type = b;
-        return;
-      }
-      if ('type' in b) {
-        a.type = b.type;
-      }
-      // readOnly: cannot become false and takes on `computed` value
-      a.readOnly = a.readOnly || Boolean(a.computed) || b.readOnly || Boolean(b.computed);
-      // computed: first in wins
-      if ('computed' in b) {
-        a.computed = a.computed || b.computed;
-      }
-      // reflectToAttribute: cannot become false
-      if ('reflectToAttribute' in b) {
-        a.reflectToAttribute = a.reflectToAttribute || b.reflectToAttribute;
-      }
-      // notify: cannot become false (only install 1x)
-      if ('notify' in b) {
-        a.notify = a.notify || b.notify;
-      }
-      // make this an array
-      if ('observer' in b) {
-        if (!a.observer) {
-          a.observer = b.observer;
-        // we have both: make array and push onto
-        } else {
-          if (!Array.isArray(a.observer)) {
-            a.observer = [a.observer];
-          }
-          a.observer.push(b.observer);
-        }
-      }
-    }
-
-    // Note, the properties in `b` are normalized before being merged
-    // into `a`. This means that given `a == {}` and `b == {foo: String}`,
-    // the result is `a == {foo: {type: String}}`.
-    function mergeElementProperties(a, b) {
-      for (let p in b) {
-        a[p] = a[p] || {};
-        mergePropertyInfo(a[p], b[p]);
-      }
-    }
-
     /* Note about construction and extension of legacy classes.
       [Changed in Q4 2018 to optimize performance.]
 
@@ -245,15 +193,43 @@
       /** @private */
       class PolymerGenerated extends Base {
 
+        // explicitly not calling super._finalizeClass
+        static _finalizeClass() {
+          // if calling via a subclass that hasn't been generated, pass through to super
+          if (!this.hasOwnProperty('generatedFrom')) {
+            super._finalizeClass();
+          } else {
+            // interleave properties and observers per behavior and `info`
+            if (behaviorList) {
+              for (let i=0, b; i < behaviorList.length; i++) {
+                b = behaviorList[i];
+                if (b.properties) {
+                  this.createProperties(b.properties);
+                }
+                if (b.observers) {
+                  this.createObservers(b.observers, b.properties);
+                }
+              }
+            }
+            if (info.properties) {
+              this.createProperties(info.properties);
+            }
+            if (info.observers) {
+              this.createObservers(info.observers, info.properties);
+            }
+            // make sure to prepare the element template
+            this._prepareTemplate();
+          }
+        }
+
         static get properties() {
           const properties = {};
           if (behaviorList) {
-            for (let i=0, b; i < behaviorList.length; i++) {
-              b = behaviorList[i];
-              mergeElementProperties(properties, b.properties);
+            for (let i=0; i < behaviorList.length; i++) {
+              Object.assign(properties, behaviorList[i].properties);
             }
           }
-          mergeElementProperties(properties, info.properties);
+          Object.assign(properties, info.properties);
           return properties;
         }
 
@@ -289,7 +265,6 @@
         /**
          * @return {void}
          */
-        // Called on element prototype
         _registered() {
           /* NOTE: `beforeRegister` is called here for bc, but the behavior
             is different than in 1.x. In 1.0, the method was called *after*
diff --git a/lib/mixins/element-mixin.html b/lib/mixins/element-mixin.html
index c9b5e189c8..ae7db9423d 100644
--- a/lib/mixins/element-mixin.html
+++ b/lib/mixins/element-mixin.html
@@ -230,14 +230,7 @@
       }
       // always add observer
       if (info.observer) {
-        if (Array.isArray(info.observer)) {
-          for (let i=0; i < info.observer.length; i++) {
-            const o = info.observer[i];
-            proto._createPropertyObserver(name, o, allProps[o]);
-          }
-        } else {
-          proto._createPropertyObserver(name, info.observer, allProps[info.observer]);
-        }
+        proto._createPropertyObserver(name, info.observer, allProps[info.observer]);
       }
       // always create the mapping from attribute back to property for deserialization.
       proto._addPropertyToAttributeMap(name);
@@ -324,12 +317,16 @@
        * @override
        * @suppress {missingProperties} Interfaces in closure do not inherit statics, but classes do
        */
-     static _finalizeClass() {
+      static _finalizeClass() {
         super._finalizeClass();
         const observers = ownObservers(this);
         if (observers) {
           this.createObservers(observers, this._properties);
         }
+        this._prepareTemplate();
+      }
+
+      static _prepareTemplate() {
         // note: create "working" template that is finalized at instance time
         let template = /** @type {PolymerElementConstructor} */ (this).template;
         if (template) {
@@ -352,7 +349,7 @@
        * @protected
        * @override
        */
-       static createProperties(props) {
+      static createProperties(props) {
         for (let p in props) {
           createPropertyFromConfig(this.prototype, p, props[p], props);
         }