Add dummy dart:html library for documentation.

lib/html/doc/interface/HashChangeEvent.dart

Index: lib/html/doc/interface/HashChangeEvent.dart
diff --git a/lib/html/doc/interface/HashChangeEvent.dart b/lib/html/doc/interface/HashChangeEvent.dart
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index 0000000000000000000000000000000000000000..c25ae875c19e481b7bd1c77473ad804c76816aab
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+++ b/lib/html/doc/interface/HashChangeEvent.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 HashChangeEvent
+interface HashChangeEvent extends Event {
+
+  /** @domName HashChangeEvent.newURL */
+  final String newURL;
+
+  /** @domName HashChangeEvent.oldURL */
+  final String oldURL;
+
+  /** @domName HashChangeEvent.initHashChangeEvent */
+  void initHashChangeEvent(String type, bool canBubble, bool cancelable, String oldURL, String newURL);
+}