Add dummy dart:html library for documentation.

lib/html/doc/interface/BeforeLoadEvent.dart

Index: lib/html/doc/interface/BeforeLoadEvent.dart
diff --git a/lib/html/doc/interface/BeforeLoadEvent.dart b/lib/html/doc/interface/BeforeLoadEvent.dart
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index 0000000000000000000000000000000000000000..2ff2b92a9ce448bd51d0f3b57c24ce1490214df4
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+++ b/lib/html/doc/interface/BeforeLoadEvent.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 BeforeLoadEvent
+interface BeforeLoadEvent extends Event {
+
+  /** @domName BeforeLoadEvent.url */
+  final String url;
+}