move workaround from CE polyfill to custom elements createdCallback

pkg/custom_element/lib/custom-elements.debug.js

Index: pkg/custom_element/lib/custom-elements.debug.js
diff --git a/pkg/custom_element/lib/custom-elements.debug.js b/pkg/custom_element/lib/custom-elements.debug.js
index 6bfbb8e44df73976264803ce36e9a50c370364cb..e3c53e07d139893e849df643800b63b6396c6381 100644
--- a/pkg/custom_element/lib/custom-elements.debug.js
+++ b/pkg/custom_element/lib/custom-elements.debug.js
@@ -962,7 +962,7 @@ var hasNative = Boolean(document.registerElement);
 // TODO(sorvell): See https://github.com/Polymer/polymer/issues/399
 // we'll address this by defaulting to CE polyfill in the presence of the SD
 // polyfill. This will avoid spamming excess attached/detached callbacks.
-// If there is a compelling need to run CE native with SD polyfill, 
+// If there is a compelling need to run CE native with SD polyfill,
 // we'll need to fix this issue.
 var useNative = !flags.register && hasNative && !window.ShadowDOMPolyfill;
 
@@ -1175,11 +1175,6 @@ if (useNative) {
       // getPrototypeOf(Element), we cannot do so when
       // we use mixin, so we install a magic reference
       customMixin(element, definition.prototype, definition.native);
-
-      // Dart note: make sure we pick up the right constructor.
-      // dart2js depends on this for dart:mirrors caching to work.
-      // See tests/html/custom/mirrors_test.dart
-      element.constructor = definition.prototype.constructor;
       element.__proto__ = definition.prototype;
     }
   }