Move frog/tests/{leg,leg_only,frog_native} to tests/compiler/.

frog/tests/leg_only/null_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.
-
-null1() {
-  return;  // implicit null;
-}
-
-null2() {
-  // Implicit null return.
-}
-
-null3() {
-  var x;  // Implicit null value.
-  return x;
-}
-
-void main() {
-  Expect.equals(null, null1());
-  Expect.equals(null, null2());
-  Expect.equals(null, null3());
-}