Remove stale files from lib/html/src.

lib/html/src/OverflowEventWrappingImplementation.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 OverflowEventWrappingImplementation extends EventWrappingImplementation implements OverflowEvent {
-  OverflowEventWrappingImplementation._wrap(ptr) : super._wrap(ptr);
-
-  /** @domName OverflowEvent.initOverflowEvent */
-  factory OverflowEventWrappingImplementation(int orient,
-      bool horizontalOverflow, bool verticalOverflow) {
-    final e = dom.document.createEvent("OverflowEvent");
-    e.initOverflowEvent(orient, horizontalOverflow, verticalOverflow);
-    return LevelDom.wrapOverflowEvent(e);
-  }
-
-  bool get horizontalOverflow() => _ptr.horizontalOverflow;
-
-  int get orient() => _ptr.orient;
-
-  bool get verticalOverflow() => _ptr.verticalOverflow;
-}