Add --no-background-compilation to tests that expect to run code in optimized form

tests/language/assign_op_test.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.
 // Dart test program for testing assign operators.
-// VMOptions=--optimization-counter-threshold=10
+// VMOptions=--optimization-counter-threshold=10 --no-background-compilation
 
 import "package:expect/expect.dart";
 
 
 class AssignOpTest {
   AssignOpTest() {}
 
   static testMain() {
     var b = 0;
     b += 1;
@@ skipping 71 matching lines @@
   }
 
   static var f;
   var instf;
 }
 main() {
   for (int i = 0; i < 20; i++) {
     AssignOpTest.testMain();
   }
 }