Add dummy dart:html library for documentation.

lib/html/doc/interface/CharacterData.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 CharacterData
+interface CharacterData extends Node {
+
+  /** @domName CharacterData.data */
+  String data;
+
+  /** @domName CharacterData.length */
+  final int length;
+
+  /** @domName CharacterData.appendData */
+  void appendData(String data);
+
+  /** @domName CharacterData.deleteData */
+  void deleteData(int offset, int length);
+
+  /** @domName CharacterData.insertData */
+  void insertData(int offset, String data);
+
+  /** @domName CharacterData.replaceData */
+  void replaceData(int offset, int length, String data);
+
+  /** @domName CharacterData.substringData */
+  String substringData(int offset, int length);
+}