move workaround from CE polyfill to custom elements createdCallback

sdk/lib/html/dart2js/html_dart2js.dart

Index: sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index f966796cd656f9c85671bcaa6fa5377e1ca38308..4021b1824e74939eb54804cbfc6dc760b386817a 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -35114,6 +35114,10 @@ _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);
   };
 }