Add the ability to change the working directory

tests/standalone/io/directory_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.
 //
 // Directory listing test.
 
 import "package:expect/expect.dart";
 import "dart:async";
 import "dart:io";
 import "dart:isolate";
@@ skipping 362 matching lines @@
           file.delete().then((ignore) {
             tempDirectory.deleteSync();
             Expect.isFalse(tempDirectory.existsSync());
           });
         });
       });
     });
   }
 
   static void testCurrent() {
-    Directory current = new Directory.current();
+    Directory current = Directory.current;
     if (Platform.operatingSystem != "windows") {
       Expect.equals("/", current.path.substring(0, 1));
     }
   }
 
   static void testFromPath() {
     var name = new File('.').fullPathSync();
     Directory current1 = new Directory(name);
     var path = new Path(name);
     Directory current2 = new Directory.fromPath(path);
@@ skipping 221 matching lines @@
   testCreateTempErrorSync();
   testCreateTempError();
   testCreateExistingSync();
   testCreateExisting();
   testCreateDirExistingFileSync();
   testCreateDirExistingFile();
   testCreateRecursive();
   testCreateRecursiveSync();
   testRename();
 }