Fixed improper construction of custom elements.

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

Index: tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate b/tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate
index 1df549ea47fd88a320ab661c4723aa1138c2c771..6cbf8834c304007bd76bf39e558b0f723418a2c6 100644
--- a/tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate
+++ b/tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate
@@ -436,6 +436,13 @@ $else
       //
       //              See https://github.com/dart-lang/sdk/issues/23666
       int creating = 0;
+
+      // If any JS code is hooked we want to call it too.
+      var oldCreatedCallback = elemProto['createdCallback'];
+      var oldAttributeChangedCallback = elemProto['attributeChangedCallback'];
+      var oldAttachedCallback = elemProto['attachedCallback'];
+      var oldDetachedCallback = elemProto['detachedCallback'];
+
       // TODO(jacobr): warning:
       elemProto['createdCallback'] = js.JsNative.withThis(($this) {
         if (_getJSClassName(reflectClass(customElementClass).superclass) != null && creating < 2) {
@@ -474,15 +481,27 @@ $else
             creating--;
           }
         }
+
+        if (oldCreatedCallback != null)
+          oldCreatedCallback.apply([], thisArg: unwrap_jso($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});