Add dummy dart:html library for documentation.

lib/html/doc/interface/CSSRule.dart

Index: lib/html/doc/interface/CSSRule.dart
diff --git a/lib/html/doc/interface/CSSRule.dart b/lib/html/doc/interface/CSSRule.dart
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+++ b/lib/html/doc/interface/CSSRule.dart
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+// 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: Do not edit - generated code.
+
+/// @domName CSSRule
+interface CSSRule {
+
+  static final int CHARSET_RULE = 2;
+
+  static final int FONT_FACE_RULE = 5;
+
+  static final int IMPORT_RULE = 3;
+
+  static final int MEDIA_RULE = 4;
+
+  static final int PAGE_RULE = 6;
+
+  static final int STYLE_RULE = 1;
+
+  static final int UNKNOWN_RULE = 0;
+
+  static final int WEBKIT_KEYFRAMES_RULE = 7;
+
+  static final int WEBKIT_KEYFRAME_RULE = 8;
+
+  /** @domName CSSRule.cssText */
+  String cssText;
+
+  /** @domName CSSRule.parentRule */
+  final CSSRule parentRule;
+
+  /** @domName CSSRule.parentStyleSheet */
+  final CSSStyleSheet parentStyleSheet;
+
+  /** @domName CSSRule.type */
+  final int type;
+}