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

client/html/src/EventSource.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 EventSourceEvents extends Events {
-  EventListenerList get error();
-  EventListenerList get message();
-  EventListenerList get open();
-}
-
-interface EventSource extends EventTarget {
-
-  static final int CLOSED = 2;
-
-  static final int CONNECTING = 0;
-
-  static final int OPEN = 1;
-
-  String get URL();
-
-  int get readyState();
-
-  void close();
-
-  EventSourceEvents get on();
-}