Fix dedupingMixin

Author: Steven Orvell

lib/utils/mixin.html

@@ -53,9 +53,9 @@
    *   mixin applications to base
    */
   Polymer.dedupingMixin = function(mixin) {
-    let cachedMixin = cachingMixin(mixin);
+    mixin = cachingMixin(mixin);
     // maintain a unique id for each mixin
-    cachedMixin.__dedupeId = ++dedupeId;
+    mixin.__dedupeId = ++dedupeId;
     return function(base) {
       let baseSet = base.__mixinSet;
       if (baseSet && baseSet[mixin.__dedupeId]) {