move workaround from CE polyfill to custom elements createdCallback

tools/dom/src/dart2js_CustomElementSupport.dart

Index: tools/dom/src/dart2js_CustomElementSupport.dart
diff --git a/tools/dom/src/dart2js_CustomElementSupport.dart b/tools/dom/src/dart2js_CustomElementSupport.dart
index b6221907be44bc2f7d480e2f2cb6f769854e458d..aa1c0372869484cefaff498db97b696d000ed05d 100644
--- a/tools/dom/src/dart2js_CustomElementSupport.dart
+++ b/tools/dom/src/dart2js_CustomElementSupport.dart
@@ -8,6 +8,10 @@ _callConstructor(constructor, interceptor) {
   return (receiver) {
     setNativeSubclassDispatchRecord(receiver, interceptor);
 
+    // Mirrors uses the constructor property to cache lookups, so we need it to
+    // be set correctly, including on IE where it is not automatically picked
+    // up from the __proto__.
+    JS('', '#.constructor = #.__proto__.constructor', receiver, receiver);
     return JS('', '#(#)', constructor, receiver);
   };
 }