Remove stale files from lib/html/src.

lib/html/src/ProgressEventWrappingImplementation.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 ProgressEventWrappingImplementation extends EventWrappingImplementation implements ProgressEvent {
-  ProgressEventWrappingImplementation._wrap(ptr) : super._wrap(ptr);
-
-  factory ProgressEventWrappingImplementation(String type, int loaded,
-      [bool canBubble = true, bool cancelable = true,
-      bool lengthComputable = false, int total = 0]) {
-    final e = dom.document.createEvent("ProgressEvent");
-    e.initProgressEvent(type, canBubble, cancelable, lengthComputable, loaded,
-        total);
-    return LevelDom.wrapProgressEvent(e);
-  }
-
-  bool get lengthComputable() => _ptr.lengthComputable;
-
-  int get loaded() => _ptr.loaded;
-
-  int get total() => _ptr.total;
-}