Add dummy dart:html library for documentation.

lib/html/doc/interface/CompositionEvent.dart

Index: lib/html/doc/interface/CompositionEvent.dart
diff --git a/lib/html/doc/interface/CompositionEvent.dart b/lib/html/doc/interface/CompositionEvent.dart
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index 0000000000000000000000000000000000000000..0130c5b16232bf5a04219d8e8e258b02caa9dfdb
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+++ b/lib/html/doc/interface/CompositionEvent.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 CompositionEvent
+interface CompositionEvent extends UIEvent {
+
+  /** @domName CompositionEvent.data */
+  final String data;
+
+  /** @domName CompositionEvent.initCompositionEvent */
+  void initCompositionEvent(String typeArg, bool canBubbleArg, bool cancelableArg, Window viewArg, String dataArg);
+}