Add VMOptions to optimize tests that need to be (Part I).

tests/language/assign_op_test.dart

Index: tests/language/assign_op_test.dart
===================================================================
--- tests/language/assign_op_test.dart	(revision 24922)
+++ tests/language/assign_op_test.dart	(working copy)
@@ -2,6 +2,7 @@
 // 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.
 // Dart test program for testing assign operators.
+// VMOptions=--optimization-counter-threshold=10
 
 import "package:expect/expect.dart";
 
@@ -89,7 +90,7 @@
   var instf;
 }
 main() {
-  for (int i = 0; i < 2000; i++) {
+  for (int i = 0; i < 20; i++) {
     AssignOpTest.testMain();
   }
 }