Remove stale files from lib/html/src.

lib/html/src/WheelEvent.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.
-
-interface WheelEvent extends UIEvent default WheelEventWrappingImplementation {
-
-  WheelEvent(int deltaX, int deltaY, Window view, int screenX, int screenY,
-      int clientX, int clientY, [bool ctrlKey, bool altKey, bool shiftKey,
-      bool metaKey]);
-
-  bool get altKey();
-
-  int get clientX();
-
-  int get clientY();
-
-  bool get ctrlKey();
-
-  bool get metaKey();
-
-  int get offsetX();
-
-  int get offsetY();
-
-  int get screenX();
-
-  int get screenY();
-
-  bool get shiftKey();
-
-  int get wheelDelta();
-
-  int get wheelDeltaX();
-
-  int get wheelDeltaY();
-
-  int get x();
-
-  int get y();
-}