Fixed improper construction of 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 {
 $!MEMBERS
@@ skipping 418 matching lines @@
       //
       //                  MyElement.created() : super.created() {
       //                    this.innerHtml = "<b>I'm an x-foo-with-markup!</b>";
       //                  }
       //
       //              sanitizing causes custom element to created recursively
       //              until stack overflow.
       //
       //              See https://github.com/dart-lang/sdk/issues/23666
       int creating = 0;
+
+      // If any JS code is hooked we want to call it too.
+      var oldCreatedCallback = elemProto['createdCallback'];
+      var oldAttributeChangedCallback = elemProto['attributeChangedCallback'];
+      var oldAttachedCallback = elemProto['attachedCallback'];
+      var oldDetachedCallback = elemProto['detachedCallback'];
+
       // TODO(jacobr): warning:
       elemProto['createdCallback'] = js.JsNative.withThis(($this) {
         if (_getJSClassName(reflectClass(customElementClass).superclass) != null && creating < 2) {
           creating++;
 
           var dartClass;
           try {
             if (extendsTag != null) {
               // If we're extending a native element then create that element.
               // Then upgrade that element to the customElementClass through
@@ skipping 18 matching lines @@
             throw e;
           } finally {
             // Need to remember the Dart class that was created for this custom so
             // return it and setup the blink_jsObject to the $this that we'll be working
             // with as we talk to blink.
             js.setDartHtmlWrapperFor($this, dartClass);
 
             creating--;
           }
         }
+
+        if (oldCreatedCallback != null)
+          oldCreatedCallback.apply([], thisArg: unwrap_jso($this));
       });
       elemProto['attributeChangedCallback'] = new js.JsFunction.withThis(($this, attrName, oldVal, newVal) {
         $this.attributeChanged(attrName, oldVal, newVal);
+
+        if (oldAttributeChangedCallback != null)
+          oldAttributeChangedCallback.apply([], thisArg: unwrap_jso($this));
       });
       elemProto['attachedCallback'] = new js.JsFunction.withThis(($this) {
         $this.attached();
+
+        if (oldAttachedCallback != null)
+          oldAttachedCallback.apply([], thisArg: unwrap_jso($this));
       });
       elemProto['detachedCallback'] = new js.JsFunction.withThis(($this) {
         $this.detached();
+
+        if (oldDetachedCallback != null)
+          oldDetachedCallback.apply([], thisArg: unwrap_jso($this));
       });
       // document.registerElement('x-foo', {prototype: elemProto, extends: extendsTag});
       var jsMap = new js.JsObject.jsify({'prototype': elemProto, 'extends': extendsTag});
       js.JsNative.callMethod(js.JsNative.getProperty(js.context, 'document'), 'registerElement', [tag, jsMap]);
     }
 $endif
   }
 
   /** *Deprecated*: use [registerElement] instead. */
   @deprecated
@@ skipping 52 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
   }
 }