Merge pull request #3408 from Polymer/3063-kschaaf-extend-template

Ensure _propertySetter is installed first. Fixes #3063

Author: Steve Orvell

src/lib/template/templatizer.html

@@ -298,6 +298,11 @@
     // values (_propertySetter back on instance once accessor is installed)
     _extendTemplate: function(template, proto) {
       var n$ = Object.getOwnPropertyNames(proto);
+      if (proto._propertySetter) {
+        // _propertySetter API may need to be copied onto the template,
+        // and it needs to come first to allow the property swizzle below
+        template._propertySetter = proto._propertySetter;
+      }
       for (var i=0, n; (i<n$.length) && (n=n$[i]); i++) {
         var val = template[n];
         var pd = Object.getOwnPropertyDescriptor(proto, n);