Add @return annotation for PROPERTY_EFFECT_TYPES getter

Author: aomarks

lib/mixins/property-effects.js

@@ -1150,6 +1150,9 @@ export const PropertyEffects = dedupingMixin(superClass => {
       this.__templateInfo;
     }
 
+    /**
+     * @return {!Object<string, string>}
+     */
     get PROPERTY_EFFECT_TYPES() {
       return TYPES;
     }