fix closure compiler error

lib/mixins/property-effects.js

@@ -1415,7 +1415,7 @@ export const PropertyEffects = dedupingMixin(superClass => {
       if (value !== node[prop] || typeof value == 'object') {
         // Note, className needs style scoping so this needs wrapping.
         if (prop === 'className') {
-          node = wrap(node);
+          node = /** @type {!Node} */(wrap(node));
         }
         node[prop] = value;
       }