Add a leg_only test suite, and make sure to run it in our presubmit script.

frog/leg/tests/3.dart

Index: frog/leg/tests/3.dart
===================================================================
--- frog/leg/tests/3.dart	(revision 1300)
+++ frog/leg/tests/3.dart	(working copy)
@@ -1,7 +0,0 @@
-// 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.
-
-void main() {
-  return 3;
-}