Fix error handling when current working directory is deleted.

tests/standalone/io/directory_chdir_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.
 //
 // Directory listing test.
 
 
 import "dart:io";
 
 import "package:async_helper/async_helper.dart";
 import "package:expect/expect.dart";
 
 testChangeDirectory() {
   asyncStart();
   new Directory("").createTemp().then((temp) {
     var initialCurrent = Directory.current;
     Directory.current = temp;
     var newCurrent = Directory.current;
     new File("111").createSync();
     var dir = new Directory(newCurrent.path + Platform.pathSeparator + "222");
     dir.createSync();
     Directory.current = dir;
     new File("333").createSync();
     Expect.isTrue(new File("333").existsSync());
     Expect.isTrue(new File("../111").existsSync());
     Directory.current = "..";
     Expect.isTrue(new File("111").existsSync());
     Expect.isTrue(new File("222/333").existsSync());
+    // Deleting the current working directory causes an error.
+    // On Windows, the deletion fails, and on non-Windows, the getter fails.
+    Expect.throws(() {
+      temp.deleteSync(recursive: true);
+      Directory.current;
+    }, (e) => e is DirectoryException);
     Directory.current = initialCurrent;
-    temp.deleteSync(recursive: true);
+    Directory.current;
+    if (temp.existsSync()) temp.deleteSync(recursive: true);
     asyncEnd();
   });
 }
 
 
 testChangeDirectoryIllegalArguments() {
   Expect.throws(() => Directory.current = 1, (e) => e is ArgumentError);
   Expect.throws(() => Directory.current = 111111111111111111111111111111111111,
                 (e) => e is ArgumentError);
   Expect.throws(() => Directory.current = true, (e) => e is ArgumentError);
   Expect.throws(() => Directory.current = [], (e) => e is ArgumentError);
   Expect.throws(() => Directory.current = new File("xxx"),
                 (e) => e is ArgumentError);
 }
 
 
 main() {
   testChangeDirectory();
   testChangeDirectoryIllegalArguments();
 }