Make Path implement (extend) Hashable, and make Path.append to an empty path work right.

runtime/bin/path_impl.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.
 
 class _Path implements Path {
   final String _path;
 
   const _Path(String source) : _path = source;
   _Path.fromNative(String source) : _path = _clean(source);
 
+  int hashCode() => _path.hashCode();
+
   static String _clean(String source) {
     switch (Platform.operatingSystem) {
       case 'windows':
         return _cleanWindows(source);
       default:
         return source;
     }
   }
 
   static String _cleanWindows(source) {
@@ skipping 151 matching lines @@
   }
 
   List<String> segments() {
     List result = _path.split('/');
     if (isAbsolute) result.removeRange(0, 1);
     if (hasTrailingSeparator) result.removeLast();
     return result;
   }
 
   Path append(String finalSegment) {
-    if (hasTrailingSeparator) {
+    if (isEmpty) {
+      return new Path(finalSegment);
+    } else if (hasTrailingSeparator) {
       return new Path('$_path$finalSegment');
     } else {
       return new Path('$_path/$finalSegment');
     }
   }
 
   String get filenameWithoutExtension() {
     var name = filename;
     if (name == '.' || name == '..') return name;
     int pos = name.lastIndexOf('.');
@@ skipping 11 matching lines @@
     if (pos < 0) return new Path('');
     while (pos > 0 && _path[pos - 1] == '/') --pos;
     return new Path((pos > 0) ? _path.substring(0, pos) : '/');
   }
 
   String get filename() {
     int pos = _path.lastIndexOf('/');
     return _path.substring(pos + 1);
   }
 }