Revert "Revert "Start of dartium custom element lifecycle event support.""

tools/dom/templates/html/impl/impl_Document.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.
 
 part of $LIBRARYNAME;
 
 @DocsEditable
 $(ANNOTATIONS)$(CLASS_MODIFIERS)class $CLASSNAME extends Node $NATIVESPEC
 {
 
@@ skipping 18 matching lines @@
   }
 
   /// Checks if [register] is supported on the current platform.
   bool get supportsRegister {
 $if DART2JS
     return JS('bool', '("register" in #)', this);
 $else
     return true;
 $endif
   }
+
+  @DomName('Document.createElement')
+  Element createElement(String tagName, [String typeExtension]) {
+$if DART2JS
+    return _createElement(tagName, typeExtension);
+$else
+    if (typeExtension != null) {
+      return _createElement(tagName, typeExtension);
+    } else {
+      // Fast-path for Dartium when typeExtension is not needed.
+      return _Utils.createElement(this, tagName);
+    }
+$endif
+  }
 }