Fix issue with observers being called twice

lib/mixins/properties-mixin.html

@@ -70,6 +70,7 @@
      */
     function superPropertiesClass(constructor) {
       const superCtor = Object.getPrototypeOf(constructor);
+
       // Note, the `PropertiesMixin` class below only refers to the class
       // generated by this call to the mixin; the instanceof test only works
       // because the mixin is deduped and guaranteed only to apply once, hence
@@ -87,10 +88,14 @@
      * @return {Object} Memoized properties object
      */
     function ownProperties(constructor) {
-      if (!constructor.hasOwnProperty(
-        JSCompiler_renameProperty('__ownProperties', constructor))) {
-        const props = constructor.properties;
-        constructor.__ownProperties = props ? normalizeProperties(props) : null;
+      if (!constructor.hasOwnProperty(JSCompiler_renameProperty('__ownProperties', constructor))) {
+        let props = null;
+
+        if (constructor.hasOwnProperty('properties') && constructor.properties) {
+          props = normalizeProperties(constructor.properties);
+        }
+
+        constructor.__ownProperties = props;
       }
       return constructor.__ownProperties;
     }

test/unit/property-effects-elements.html

@@ -1038,4 +1038,27 @@
     this.xChanged = sinon.spy();
   }
 });
-</script>
+</script>
+
+<script>
+class SuperObserverElement extends Polymer.Element {
+  static get is() { return 'super-observer-element'; }
+  static get properties() {
+    return {
+      prop: {
+        value: 'String',
+        observer(value, oldValue) {
+          this.__observerCalled++;
+        }
+      }
+    }
+  }
+}
+SuperObserverElement.prototype.__observerCalled = 0;
+customElements.define(SuperObserverElement.is, SuperObserverElement);
+
+class SubObserverElement extends SuperObserverElement {
+  static get is() { return 'sub-observer-element'; }
+}
+customElements.define(SubObserverElement.is, SubObserverElement);
+</script>

test/unit/property-effects.html

@@ -409,6 +409,16 @@
     }
   });
 
+  suite('observer inheritance', function() {
+    setup(function() {
+      el = document.createElement('sub-observer-element');
+      document.body.appendChild(el);
+    });
+
+    test('does not invoke observer twice', function() {
+      assert.equal(el.__observerCalled, 1);
+    });
+  });
 });
 
 suite('computed bindings with dynamic functions', function() {