Revert Promise<void> changes.

externs/closure-types.js

@@ -33,7 +33,7 @@ Polymer_PropertiesChanged.prototype._addPropertyToAttributeMap = function(proper
 */
 Polymer_PropertiesChanged.prototype._definePropertyAccessor = function(property, readOnly){};
 /**
-* @return {void|!Promise<void>}
+* @return {void}
 */
 Polymer_PropertiesChanged.prototype.ready = function(){};
 /**
@@ -379,7 +379,7 @@ Polymer_PropertyEffects.prototype.__templateInfo;
 Polymer_PropertyEffects.prototype._stampTemplate = function(template){};
 /**
 * @override
-* @return {void|!Promise<void>}
+* @return {void}
 */
 Polymer_PropertyEffects.prototype.ready = function(){};
 /**

lib/mixins/properties-changed.js

@@ -179,14 +179,13 @@ export const PropertiesChanged = dedupingMixin(
      * bindings.  `super.ready()` must be called to ensure the data system
      * becomes enabled.
      *
-     * @return {void|!Promise<void>} void
+     * @return {void}
      * @public
      * @override
      */
     ready() {
       this.__dataReady = true;
       this._flushProperties();
-      return undefined;
     }
 
     /**