Remove stale files from lib/html/src.

lib/html/src/UIEvent.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 UIEvent extends Event default UIEventWrappingImplementation {
-
-  UIEvent(String type, Window view, int detail, [bool canBubble,
-      bool cancelable]);
-
-  int get charCode();
-
-  int get detail();
-
-  int get keyCode();
-
-  int get layerX();
-
-  int get layerY();
-
-  int get pageX();
-
-  int get pageY();
-
-  Window get view();
-
-  int get which();
-}