Remove stale files from lib/html/src.

lib/html/src/PopStateEvent.dart

Index: lib/html/src/PopStateEvent.dart
diff --git a/lib/html/src/PopStateEvent.dart b/lib/html/src/PopStateEvent.dart
deleted file mode 100644
index 470db3b5b6e6289694d891eb6bcdd38e4d975750..0000000000000000000000000000000000000000
--- a/lib/html/src/PopStateEvent.dart
+++ /dev/null
@@ -1,10 +0,0 @@
-// 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 PopStateEvent extends Event default PopStateEventWrappingImplementation {
-
-  PopStateEvent(String type, Object state, [bool canBubble, bool cancelable]);
-
-  String get state();
-}