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

tests/language/load_indexed_constant_test.dart

 // Copyright (c) 2013, 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.
 // Test constant propgation of load-indexed operations
-// VMOptions=--optimization-counter-threshold=10
+// VMOptions=--optimization-counter-threshold=10 --no-background-compilation
 
 import "package:expect/expect.dart";
 
 
 main() {
   Expect.equals(101, stringIndexedLoad());
   Expect.equals(102, arrayIndexedLoad());
   for (int i = 0; i < 20; i++) {
     stringIndexedLoad();
     arrayIndexedLoad();
   }
   Expect.equals(101, stringIndexedLoad());
   Expect.equals(102, arrayIndexedLoad());
 }
 
 stringIndexedLoad() => ("Hello").codeUnitAt(1);
 arrayIndexedLoad() => (const [101, 102, 103])[1];