dart:io | Add Link.update, and change Link.updateSync.

tests/standalone/io/link_async_test.dart

 // Copyright (c) 2013, 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.
 
 import "package:expect/expect.dart";
 import "package:path/path.dart";
 import "dart:async";
 import "dart:io";
 import "dart:isolate";
 
@@ skipping 163 matching lines @@
     })
     .then((link) {
       link2 = link;
       Expect.isFalse(link1.existsSync());
       Expect.isTrue(link2.existsSync());
       return link2.delete();
     })
     .then((_) => Expect.isFalse(link2.existsSync()));
   }
 
+  Future testUpdate(String base , String target1, String target2) {
+    Link link1;
+    return new Link(join(base, 'c')).create(target1)
+    .then((link) {
+      link1 = link;
+      Expect.isTrue(link1.existsSync());
+      return link1.update(target2);
+    })
+    .then((Link link) {
+      Expect.isTrue(link1.existsSync());
+      Expect.isTrue(link.existsSync());
+      return FutureExpect.equals(target2, link.target())
+      .then((_) => FutureExpect.equals(target2, link1.target()))
+      .then((_) => link.delete());
+    })
+    .then((_) => Expect.isFalse(link1.existsSync()));
+  }
+
   return new Directory('').createTemp().then((baseDir) {
     String base = baseDir.path;
     var targetsFutures = [];
     targetsFutures.add(new Directory(join(base, 'a')).create());
     if (Platform.isWindows) {
       // Currently only links to directories are supported on Windows.
       targetsFutures.add(
           new Directory(join(base, 'b')).create());
     } else {
       targetsFutures.add(new File(join(base, 'b')).create());
     }
     return Future.wait(targetsFutures).then((targets) {
       return testRename(base, targets[0].path)
       .then((_) => testRename(base, targets[1].path))
+      .then((_) => testUpdate(base, targets[0].path, targets[1].path))
       .then((_) => baseDir.delete(recursive: true));
     });
   });
 }
 
 Future testDirectoryListing(String base, Directory baseDir) {
   Map makeExpected(bool recursive, bool followLinks) {
     Map expected = new Map();
     expected['target'] = 'Directory';
     expected['link'] = followLinks ? 'Directory' : 'Link';
@@ skipping 43 matching lines @@
   return Future.wait(futures);
 }
 
 main() {
   ReceivePort keepAlive = new ReceivePort();
   testCreate()
     .then((_) => testCreateLoopingLink())
     .then((_) => testRename())
     .then((_) => keepAlive.close());
 }