Update to new Webkit additions for custom elements

tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // WARNING: Do not edit - generated code.
 
 part of $LIBRARYNAME;
 
 $(ANNOTATIONS)$(NATIVESPEC)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS {
 $!MEMBERS
@@ skipping 361 matching lines @@
       int creating = 0; // TODO(jacobr): I think I broke thise case. Will fix monday.
 
       // If any JS code is hooked we want to call it too.
       var oldCreatedCallback = js.JsNative.getProperty(elemProto, 'createdCallback');
       var oldAttributeChangedCallback = js.JsNative.getProperty(elemProto, 'attributeChangedCallback');
       var oldAttachedCallback = js.JsNative.getProperty(elemProto, 'attachedCallback');
       var oldDetachedCallback = js.JsNative.getProperty(elemProto, 'detachedCallback');
 
       js.JsNative.setProperty(elemProto, 'createdCallback', js.allowInteropCaptureThis(($this) {
         // The created callback has already been called by the very act of passing a JS
         // custom element from JS to Dart.

[Jacob, 2016/04/25 14:54:12]

this previous comment says something conflicting and incorrect.
Add a comment here describing why we have to call this.

+
+        //  Make element's interceptor a CustomElementClass.
+        _blink.Blink_Utils.setInstanceInterceptorCustomUpgrade($this);
+
         if (oldCreatedCallback != null)
           oldCreatedCallback.apply([], thisArg: $this);
       }));
       js.JsNative.setProperty(elemProto, 'attributeChangedCallback', js.allowInteropCaptureThis(($this, attrName, oldVal, newVal) {
         $this.attributeChanged(attrName, oldVal, newVal);
 
         if (oldAttributeChangedCallback != null)
           oldAttributeChangedCallback.apply([], thisArg: $this);
       }));
       js.JsNative.setProperty(elemProto, 'attachedCallback', js.allowInteropCaptureThis(($this) {
@@ skipping 73 matching lines @@
   /// parameter must be provided.
   @Experimental()
   ElementUpgrader createElementUpgrader(Type type, {String extendsTag}) {
 $if DART2JS
     return new _JSElementUpgrader(this, type, extendsTag);
 $else
     return new _VMElementUpgrader(this, type, extendsTag);
 $endif
   }
 }