Fix dart2js warnings and add test to ensure it stays clean.

pkg/testing/lib/src/test_dart/path.dart

 // Copyright (c) 2012, 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.
 
-library legacy_path;
+library test_dart_copy.legacy_path;
 
 import 'dart:io';
 import 'dart:math';
 
 // TODO: Remove this class, and use the URI class for all path manipulation.
 class Path {
   final String _path;
   final bool isWindowsShare;
 
   Path(String source)
@@ skipping 286 matching lines @@
     while (pos > 0 && _path[pos - 1] == '/') --pos;
     var dirPath = (pos > 0) ? _path.substring(0, pos) : '/';
     return new Path._internal(dirPath, isWindowsShare);
   }
 
   String get filename {
     int pos = _path.lastIndexOf('/');
     return _path.substring(pos + 1);
   }
 }