Add dummy dart:html library for documentation.

lib/html/doc/interface/CSSValue.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 CSSValue
+interface CSSValue {
+
+  static final int CSS_CUSTOM = 3;
+
+  static final int CSS_INHERIT = 0;
+
+  static final int CSS_PRIMITIVE_VALUE = 1;
+
+  static final int CSS_VALUE_LIST = 2;
+
+  /** @domName CSSValue.cssText */
+  String cssText;
+
+  /** @domName CSSValue.cssValueType */
+  final int cssValueType;
+}