Add dummy dart:html library for documentation.

lib/html/doc/interface/CSSMediaRule.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 CSSMediaRule
+interface CSSMediaRule extends CSSRule {
+
+  /** @domName CSSMediaRule.cssRules */
+  final CSSRuleList cssRules;
+
+  /** @domName CSSMediaRule.media */
+  final MediaList media;
+
+  /** @domName CSSMediaRule.deleteRule */
+  void deleteRule(int index);
+
+  /** @domName CSSMediaRule.insertRule */
+  int insertRule(String rule, int index);
+}