Add simple directory listing test.

runtime/tests/dart/src/DirectoryTest.dart

+// Copyright (c) 2011, 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.
+
+class DirectoryTest {
+  static void testListing() {
+    bool listedSomething = false;
+    Directory directory = new Directory.open(".");
+
+    directory.setDirHandler((dir) {
+      listedSomething = true;
+    });
+
+    directory.setFileHandler((f) {
+      listedSomething = true;
+    });
+
+    directory.setDoneHandler((completed) {

[Søren Gjesse, 2011/10/10 14:58:00]

Perhaps format like 

directory.setDoneHandler(
    (completed) {
      ...
    }

[Mads Ager (google), 2011/10/10 15:07:37]

I like the current style better. I find the indentation weird when doing:

directory.setDirHandler(
    (dir) {
      listedSomething = true;
    });

+      Expect.isTrue(completed, "directory listing did not complete");
+      Expect.isTrue(listedSomething, "empty directory");
+      directory.close();
+    });
+
+    directory.setDirErrorHandler((dir) {
+      Expect.fail("error listing directory");
+    });
+
+    directory.list();
+  }
+
+  static void testMain() {
+    testListing();
+  }
+}
+
+main() {
+  DirectoryTest.testMain();
+}