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

runtime/bin/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.
 
 /**
  * A Path, which is a String interpreted as a sequence of path segments,
  * which are strings, separated by forward slashes.
  * Paths are immutable wrappers of a String, that offer member functions for
  * useful path manipulations and queries.  Joining of paths and normalization
  * interpret '.' and '..' in the usual way.
  */
-interface Path default _Path {
+interface Path extends Hashable default _Path {
   /**
    * Creates a Path from the String [source].  [source] is used as-is, so if
    * the string does not consist of segments separated by forward slashes, the
    * behavior may not be as expected.  Paths are immutable, and constant
    * Path objects may be constructed from constant Strings.
    */
   const Path(String source);
 
   /**
    * Creates a Path from a String that uses the native filesystem's conventions.
@@ skipping 143 matching lines @@
 
   /**
    * The part of [filename] after the last '.', or '' if [filename]
    * contains no '.'.  If [filename] is '.' or '..', returns ''.
    *
    *     new Path('tiger.svg').extension == 'svg'
    *     new Path('/src/dart/dart_secrets').extension == ''
    */
   String get extension();
 }