Add the ability to change the working directory

sdk/lib/io/directory_impl.dart

Index: sdk/lib/io/directory_impl.dart
diff --git a/sdk/lib/io/directory_impl.dart b/sdk/lib/io/directory_impl.dart
index 3aaf2194d51ee9a1cefb4b0204dfa2c0d19eb01b..72ca527dcab23725d1c45612d4d47ec5191b60af 100644
--- a/sdk/lib/io/directory_impl.dart
+++ b/sdk/lib/io/directory_impl.dart
@@ -18,9 +18,9 @@ class _Directory implements Directory {
 
   _Directory(String this._path);
   _Directory.fromPath(Path path) : this(path.toNativePath());
-  _Directory.current() : this(_current());
 
   external static String _current();
+  external static _setCurrent(String path);
   external static _createTemp(String template);
   external static int _exists(String path);
   external static _create(String path);
@@ -29,6 +29,18 @@ class _Directory implements Directory {
   external static List _list(String path, bool recursive, bool followLinks);
   external static SendPort _newServicePort();
 
+  static Directory get current => new _Directory(_current());
+
+  static void set current(path) {
+    if (path is Directory) path = path.path;
+    var result = _setCurrent(path);
+    if (result is ArgumentError) throw result;
+    if (result is OSError) {
+      throw new DirectoryIOException(
+          "Setting current working directory failed", path, result);
+    }
+  }
+
   Future<bool> exists() {
     _ensureDirectoryService();
     List request = new List(2);