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

tests/language/vm/string_polymorphic_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.
 // Tests that the VM does not crash on weird corner cases of class Math.
-// VMOptions=--optimization_counter_threshold=10
+// VMOptions=--optimization_counter_threshold=10 --no-background_compilation
 
 import 'package:expect/expect.dart';
 
 test1(String a, String b) {
   return a == b;
 }
 
 var LEN = 500;
 
 var ITER = 100000 / LEN;
 
 measure(fn, a, b) {
   for (var i = 0; i < ITER; i++) {
     Expect.equals(true, fn(a, b));
   }
 }
 
 main() {
   var n = LEN;
   StringBuffer s = new StringBuffer();
   for (var i=0; i < n; ++i) s.write("A");
   String t = s.toString();
   String u = s.toString();
   String v = s.toString() + "\u1234";
   String w = s.toString() + "\u1234";
   for (var i=0; i<10; i++) measure(test1, t, u);
   for (var i=0; i<10; i++) measure(test1, v, w);
 }