Add the ability to change the working directory

sdk/lib/io/directory.dart

Index: sdk/lib/io/directory.dart
diff --git a/sdk/lib/io/directory.dart b/sdk/lib/io/directory.dart
index 8c0ad5abd05cf07d234a26a0e0a001a8241cd45d..59dc529522944d1876c62d2dacb21363fc5b63df 100644
--- a/sdk/lib/io/directory.dart
+++ b/sdk/lib/io/directory.dart
@@ -26,7 +26,29 @@ abstract class Directory extends FileSystemEntity {
    * Creates a directory object pointing to the current working
    * directory.
    */
-  factory Directory.current() => new _Directory.current();
+  static Directory get current => _Directory.current;
+
+  /**
+   * Sets the current working directory of the Dart process including
+   * all running isolates. The new value set can be either a [Directory]
+   * or a [String].
+   *
+   * The new value is passed to the OS's system call unchanged, so a
+   * relative path passed as the new working directory will be
+   * resolved by the OS.
+   *
+   * Note that setting the current working directory is a synchronous
+   * operation and that it changes the the working directory of *all*
+   * isolates.
+   *
+   * Use this with care - especially when working with asynchronous
+   * operations and multiple isolates. Changing the working directory,
+   * while asynchronous operations are pending or when other isolates
+   * are working with the file system, can lead to unexpected results.
+   */
+  static void set current(path) {
+    _Directory.current = path;
+  }
 
   /**
    * Check whether a directory with this name already exists. Returns