Add dummy dart:html library for documentation.

lib/html/doc/interface/ShadowRoot.dartdoc

+// 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:
+//   This file contains documentation that is merged into the real source.
+//   Do not make code changes here.
+
+/// @domName ShadowRoot
+interface ShadowRoot extends DocumentFragment default _ShadowRootFactoryProvider {
+
+  ShadowRoot(Element host);
+
+  /** @domName ShadowRoot.activeElement */
+  final Element activeElement;
+
+  /** @domName ShadowRoot.applyAuthorStyles */
+  bool applyAuthorStyles;
+
+  /** @domName ShadowRoot.host */
+  final Element host;
+
+  /** @domName ShadowRoot.innerHTML */
+  String innerHTML;
+
+  /** @domName ShadowRoot.getElementById */
+  Element getElementById(String elementId);
+
+  /** @domName ShadowRoot.getElementsByClassName */
+  NodeList getElementsByClassName(String className);
+
+  /** @domName ShadowRoot.getElementsByTagName */
+  NodeList getElementsByTagName(String tagName);
+
+  /** @domName ShadowRoot.getElementsByTagNameNS */
+  NodeList getElementsByTagNameNS(String namespaceURI, String localName);
+
+  /** @domName ShadowRoot.getSelection */
+  DOMSelection getSelection();
+}