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

tests/language/string_charcode_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
 
 import "package:expect/expect.dart";
 
 main() {
   for (int i = 0; i < 20; i++) {
     Expect.isTrue(moo("x"));
     Expect.isFalse(moo("X"));
     Expect.isFalse(moo("xx"));
     Expect.isTrue(mooRev("x"));
     Expect.isFalse(mooRev("X"));
@@ skipping 36 matching lines @@
 
 gooRev(a, j) {
   return j == a[1];
 }
 
 // Check constant folding.
 hooRev(a) {
   return ("Hello")[0] == a[1];
 }