Always return true for ShadowRoot.supported.

tools/dom/src/native_DOMImplementation.dart

 // 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 html;
 
 class _Utils {
   static double dateTimeToDouble(DateTime dateTime) =>
       dateTime.millisecondsSinceEpoch.toDouble();
   static DateTime doubleToDateTime(double dateTime) {
@@ skipping 53 matching lines @@
 
   static makeUnimplementedError(String fileName, int lineNo) {
     return new UnsupportedError('[info: $fileName:$lineNo]');
   }
 
   static window() native "Utils_window";
   static forwardingPrint(String message) native "Utils_forwardingPrint";
   static void spawnDomFunction(Function f, int replyTo) native "Utils_spawnDomFunction";
   static void spawnDomUri(String uri, int replyTo) native "Utils_spawnDomUri";
   static int _getNewIsolateId() native "Utils_getNewIsolateId";
-  static bool shadowRootSupported(Document document) native "Utils_shadowRootSupported";
 }
 
 class _NPObject extends NativeFieldWrapperClass1 {
   _NPObject.internal();
   static _NPObject retrieve(String key) native "NPObject_retrieve";
   property(String propertyName) native "NPObject_property";
   invoke(String methodName, [List args = null]) native "NPObject_invoke";
 }
 
 class _DOMWindowCrossFrame extends NativeFieldWrapperClass1 implements
@@ skipping 165 matching lines @@
   }
 
   _send(msg) {
     _sendToHelperIsolate(msg, _sendPort);
   }
 }
 
 get _pureIsolateTimerFactoryClosure =>
     ((int milliSeconds, void callback(Timer time), bool repeating) =>
         new _PureIsolateTimer(milliSeconds, callback, repeating));