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

tests/language/string_intrinsics_test.dart

 // Copyright (c) 2012, 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.
 // Replace with shared test once interface issues clarified.
 // Test various String intrinsics
-// VMOptions=--optimization-counter-threshold=10
+// VMOptions=--optimization-counter-threshold=10 --no-background-compilation
 
 import "package:expect/expect.dart";
 
 main() {
   var oneByte = "Hello world";
   var empty = "";
   for (int i = 0; i < 20; i++) {
     Expect.equals(11, testLength(oneByte));
     Expect.equals(0, testLength(empty));
     Expect.isFalse(testIsEmpty(oneByte));
     Expect.isTrue(testIsEmpty(empty));
   }
 }
 
 
 testLength(s) {
   return s.length;
 }
 
 
 testIsEmpty(s) {
   return s.isEmpty;
 }