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

tests/corelib/regress_r21715_test.dart

 // Copyright (c) 2014, 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
+// VMOptions=--optimization_counter_threshold=5 --no-background_compilation
 
 import "package:expect/expect.dart";
 
 sll(x, shift) => x << shift;
 
 main() {
   for (int i = 0; i < 10; i++) {
     var x = 0x50000000;
     var shift = 34;
     Expect.equals(sll(x, shift), 0x14000000000000000);
   }
 }