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

client/html/src/EventSourceWrappingImplementation.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 EventSourceEventsImplementation extends EventsImplementation implements EventSourceEvents {
-  EventSourceEventsImplementation._wrap(_ptr) : super._wrap(_ptr);
-
-  EventListenerList get error() => _get('error');
-  EventListenerList get message() => _get('message');
-  EventListenerList get open() => _get('open');
-}
-
-class EventSourceWrappingImplementation extends EventTargetWrappingImplementation implements EventSource {
-  EventSourceWrappingImplementation._wrap(ptr) : super._wrap(ptr);
-
-  String get URL() => _ptr.URL;
-
-  int get readyState() => _ptr.readyState;
-
-  void close() {
-    _ptr.close();
-  }
-
-  EventSourceEvents get on() {
-    if (_on === null) {
-      _on = new EventSourceEventsImplementation._wrap(_ptr);
-    }
-    return _on;
-  }
-}