Sync memoized properties getter with 3.x version.

Author: kevinpschaaf

lib/mixins/properties-mixin.html

@@ -93,8 +93,12 @@
       if (!constructor.hasOwnProperty(JSCompiler_renameProperty('__ownProperties', constructor))) {
         let props = null;
 
-        if (constructor.hasOwnProperty(JSCompiler_renameProperty('properties', constructor)) && constructor.properties) {
-          props = normalizeProperties(constructor.properties);
+        if (constructor.hasOwnProperty(JSCompiler_renameProperty('properties', constructor))) {
+          const properties = constructor.properties;
+          
+          if (properties) {
+            props = normalizeProperties(properties);
+          }
         }
 
         constructor.__ownProperties = props;
@@ -117,9 +121,12 @@
        * @suppress {missingProperties} Interfaces in closure do not inherit statics, but classes do
        */
       static get observedAttributes() {
-        Polymer.telemetry.register(this.prototype);
-        const props = this._properties;
-        return props ? Object.keys(props).map(p => this.attributeNameForProperty(p)) : [];
+        if (!this.hasOwnProperty('__observedAttributes')) {
+          Polymer.telemetry.register(this.prototype);
+          const props = this._properties;
+          this.__observedAttributes = props ? Object.keys(props).map(p => this.attributeNameForProperty(p)) : [];
+        }
+        return this.__observedAttributes;
       }
 
       /**