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

tests/language/src/ClassOverrideNegativeTest.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.
-// Check fail because incompatible overriding method
+
+// Test fails in checked mode because incompatible overriding method
+// VMOptions=--enable_type_checks
 
 class A {
   foo() {}
 }
 
 class B extends A {
   foo(a) { }
 }
 
 class ClassOverrideNegativeTest {
   static testMain() {
   }
 }
 
 main() {
   ClassOverrideNegativeTest.testMain();
 }