Add Timer implementation for dart2js. Also moves tests to appropriate location.

tests/isolate/timer_not_available_test.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.
+
+#library('timer_not_available');
+
+#import("dart:isolate");
+
+main() {
+  bool failed = false;
+  try {
+    new Timer(0, (_) { });
+  } catch (UnsupportedOperationException e) {
+    failed = true;
+  }
+  Expect.isTrue(failed);
+  failed = false;
+  try {
+    var t = new Timer.repeating(10, (_) { });
+    t.cancel();
+  } catch (UnsupportedOperationException e) {
+    failed = true;
+  }
+  Expect.isTrue(failed);
+}