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);

[blois, 2014/02/04 00:02:06]

Should this be in setNativeSubclassDispatchRecord? Seems like it's part of the
work that it's already doing.

[Jennifer Messerly, 2014/02/04 00:28:18]

Hmmm, that's a good question. I don't think so. setNativeSubclassDispatchRecord
is part of dart2js in sdk/lib/_internal, whereas this code is specific to
dart2js custom elements support (and the polyfill only, for that matter). So 
dart2js_CustomElementSupport.dart seems like the right place for this code to
live. IMHO.

[sra1, 2014/02/05 00:40:43]

Lets keep 'setNativeSubclassDispatchRecord' setting just the dispatch record for
now.

     return JS('', '#(#)', constructor, receiver);
   };
 }