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

tests/language/mega_load_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.
 // Test megamorphic, but single target field load.
-// VMOptions=--optimization-counter-threshold=10
+// VMOptions=--optimization-counter-threshold=10 --no-background-compilation
 
 import "package:expect/expect.dart";
 
 class Base {
   var f;
 }
 
 class A extends Base {
   A() { this.f = 0; }
 }
@@ skipping 131 matching lines @@
   }
 }
 
 main() {
   var list = allocateObjects();
   // Make sure the optimizer triggers the compilation of callThemAll.
   for (var i = 0; i < 20; i++) {
     callThemAll(list);
   }
 }