Add dummy dart:html library for documentation.

lib/html/doc/interface/Text.dartdoc

+// Copyright (c) 2011, 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 Text
+interface Text extends CharacterData default _TextFactoryProvider {
+
+  Text(String data);
+
+  /** @domName Text.wholeText */
+  final String wholeText;
+
+  /** @domName Text.replaceWholeText */
+  Text replaceWholeText(String content);
+
+  /** @domName Text.splitText */
+  Text splitText(int offset);
+}