Verify method overrides in checked mode only (fix issue 69).

tests/language/src/ClassOverrideNegativeTest.dart

Index: tests/language/src/ClassOverrideNegativeTest.dart
===================================================================
--- tests/language/src/ClassOverrideNegativeTest.dart	(revision 488)
+++ tests/language/src/ClassOverrideNegativeTest.dart	(working copy)
@@ -1,8 +1,10 @@
 // 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.
-// Check fail because incompatible overriding method
 
+// Test fails in checked mode because incompatible overriding method
+// VMOptions=--enable_type_checks
+
 class A {
   foo() {}
 }