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

tests/language/optimized_isempty_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.
-// VMOptions=--optimization-counter-threshold=10 --no-use-osr
+// VMOptions=--optimization-counter-threshold=10 --no-use-osr --no-background-compilation
 
 // Test optimization and polymorphic inlining of String.isEmpty.
 
 import "package:expect/expect.dart";
 
 test(s) => s.isEmpty;
 
 main() {
   var x = "abc";
   var y = [123, 12345, 765];
   Expect.equals(false, test(x));
   Expect.equals(false, test(y));
   for (var i=0; i<20; i++) test(x);
   Expect.equals(false, test(x));
   Expect.equals(false, test(y));
 }