Remove stale files from lib/html/src.

lib/html/src/CompositionEventWrappingImplementation.dart

-// Copyright (c) 2011, 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.
-
-class CompositionEventWrappingImplementation extends UIEventWrappingImplementation implements CompositionEvent {
-  CompositionEventWrappingImplementation._wrap(ptr) : super._wrap(ptr);
-
-  factory CompositionEventWrappingImplementation(String type, Window view,
-      String data, [bool canBubble = true, bool cancelable = true]) {
-    final e = dom.document.createEvent("CompositionEvent");
-    e.initCompositionEvent(type, canBubble, cancelable, LevelDom.unwrap(view),
-        data);
-    return LevelDom.wrapCompositionEvent(e);
-  }
-
-  String get data() => _ptr.data;
-}