Fixed NullPointerException in Timer.

runtime/tests/dart/src/TimerCancel2Test.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.
+
+// Test that a timeout handler can cancel itself.
+class TimerCancel2Test {
+  static void testSelfCancel() {
+    var cancelTimer;
+
+    void cancelHandler(Timer timer) {
+      cancelTimer.cancel();
+    }
+
+    cancelTimer = new Timer(cancelHandler, 1, true);
+  }
+
+  static void testMain() {
+    testSelfCancel();
+  }
+}
+
+main() {
+  TimerCancel2Test.testMain();
+}