Add a separate test verifying that the optimizer does not trip over debug break.

tests/language/vm/debug_break_vm_test.dart

 // Copyright (c) 2015, 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.
-// VMOptions=--optimization-counter-threshold=5
 
 // A debug break is not valid Dart syntax unless --enable-debug-break.
 
 test(i) {
   break "outside_loop";  /// 02: compile-time error
   do {
     if (i > 15) {
       break "inside_loop";  /// 03: compile-time error
     }
   } while (false);
 }
 
 void main() {
   break "gdb";   /// 01: compile-time error
   for (var i = 0; i < 20; i++) {
     test(i);
   }
 }