Add exists, create and delete to directory implementation on Linux and Mac.

tests/standalone/src/DirectoryTest.dart

Index: tests/standalone/src/DirectoryTest.dart
diff --git a/tests/standalone/src/DirectoryTest.dart b/tests/standalone/src/DirectoryTest.dart
index ddf5b1bfd267fa80b69eddab15457e7eeb810441..3892cd029556c878d3503eb23d67a196d2312a9a 100644
--- a/tests/standalone/src/DirectoryTest.dart
+++ b/tests/standalone/src/DirectoryTest.dart
@@ -33,8 +33,19 @@ class DirectoryTest {
     Expect.isFalse(listedSomething);
   }
 
+  static void testExistsCreateDelete() {
+    // TODO(ager): This should be creating temporary directories.
+    Directory d = new Directory("____DIRECTORY_TEST_DIRECTORY____");
+    Expect.isFalse(d.exists());
+    d.create();
+    Expect.isTrue(d.exists());
+    d.delete();
+    Expect.isFalse(d.exists());
+  }
+
   static void testMain() {
     testListing();
+    testExistsCreateDelete();
   }
 }