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

tests/standalone/io/path_test.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.
 //
 // Test the Path class in dart:io.
 
 #import("dart:io");
 
 void main() {
   testBaseFunctions();
@@ skipping 139 matching lines @@
   testJoin('a/b/c', '../../d/', 'a/d/');
 
   void testAppend(String a, String b, String c) {
     Expect.equals(new Path(a).append(b).toString(), c);
   }
 
   testAppend('/a/b', 'c', '/a/b/c');
   testAppend('a/b/', 'cd', 'a/b/cd');
   testAppend('.', '..', './..');
   testAppend('a/b', '/c/d', 'a/b//c/d');
+  testAppend('', 'foo/bar', 'foo/bar');
+  testAppend('/foo', '', '/foo/');
 
   // .join can only join a relative path to a path.
   // It cannot join an absolute path to a path.
   Expect.throws(() => new Path('/a/b/').join(new Path('/c/d')));
 
   // Test Path.relativeTo.
   Expect.equals('c/d',
                 new Path('/a/b/c/d').relativeTo(new Path('/a/b')).toString());
   Expect.equals('c/d',
                 new Path('/a/b/c/d').relativeTo(new Path('/a/b/')).toString());
   Expect.equals('.',
                 new Path('/a').relativeTo(new Path('/a')).toString());
 
   // Not implemented yet.  Should return new Path('../b/c/d/').
   Expect.throws(() =>
                 new Path('b/c/d/').relativeTo(new Path('a/')));
   // Should always throw - no relative path can be constructed.
   Expect.throws(() =>
                 new Path('a/b').relativeTo(new Path('../../d')));
 }