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

tests/language/optimized_string_charat_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.
-// VMOptions=--optimization-counter-threshold=10 --no-use-osr
+// VMOptions=--optimization-counter-threshold=10 --no-use-osr --no-background-compilation
 
 import "package:expect/expect.dart";
 
 // Test optimized [] on strings.
 
 var a = "abc";
 var b = "øbc";
 var c = new String.fromCharCodes([123, 456, 789]);
 
 test_charat(s, i) {
@@ skipping 44 matching lines @@
 
   Expect.equals(new String.fromCharCodes([123]), test_charat(c, 0));
   for (var i = 0; i < 20; i++) test_charat(c, 0);
   Expect.equals(new String.fromCharCodes([123]), test_charat(c, 0));
   Expect.equals(new String.fromCharCodes([456]), test_charat(c, 1));
   Expect.equals(new String.fromCharCodes([789]), test_charat(c, 2));
   Expect.throws(() => test_charat(c, 3));
 
 
 }