Revert r16154: some browser tests in checked mode fail.

tests/language/null_typedef_test.dart

Index: tests/language/null_typedef_test.dart
===================================================================
--- tests/language/null_typedef_test.dart	(revision 16156)
+++ tests/language/null_typedef_test.dart	(working copy)
@@ -1,13 +0,0 @@
-// Copyright (c) 2012, 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.
-
-var a = [null];
-
-typedef void func();
-
-main() {
-  func local = a[0];
-  Expect.isFalse(local is func);
-  a[0] = 42;
-}