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

tests/language/vm/debug_break_enabled_vm_test.dart

Index: tests/language/vm/debug_break_enabled_vm_test.dart
===================================================================
--- tests/language/vm/debug_break_enabled_vm_test.dart	(revision 0)
+++ tests/language/vm/debug_break_enabled_vm_test.dart	(working copy)
@@ -0,0 +1,21 @@
+// 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 --enable-debug-break
+
+// Verify that the optimizer does not trip over the debug break (StopInstr).
+
+test(i) {
+  if (i.isOdd) {
+    break "never_hit";
+  }
+/* Unfortunately, it is not possible to specify a "crash" outcome:
+  if (i == 18) break "hit";   / / / 01: crash

[Ivan Posva, 2015/04/20 22:05:14]

As discussed we can execute this piece of code on the VM by setting a Crash, OK
expectation in the status files.

[regis, 2015/04/20 23:17:58]

Good idea. Done.

+*/
+}
+
+void main() {
+  for (var i = 0; i < 20; i += 2) {
+    test(i);
+  }
+}