Add dummy dart:html library for documentation.

lib/html/doc/interface/StyleSheet.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 StyleSheet
+interface StyleSheet {
+
+  /** @domName StyleSheet.disabled */
+  bool disabled;
+
+  /** @domName StyleSheet.href */
+  final String href;
+
+  /** @domName StyleSheet.media */
+  final MediaList media;
+
+  /** @domName StyleSheet.ownerNode */
+  final Node ownerNode;
+
+  /** @domName StyleSheet.parentStyleSheet */
+  final StyleSheet parentStyleSheet;
+
+  /** @domName StyleSheet.title */
+  final String title;
+
+  /** @domName StyleSheet.type */
+  final String type;
+}