Remove files that have already been ported or do not need to be ported.

client/html/src/EventTarget.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 EventListenerList {
-  EventListenerList add(EventListener handler, [bool useCapture]);
-
-  EventListenerList remove(EventListener handler, [bool useCapture]);
-
-  bool dispatch(Event evt);
-}
-
-interface Events {
-  EventListenerList operator [](String type);
-}
-
-interface EventTarget {
-  Events get on();
-}
-