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

tests/language/optimized_string_charcodeat_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
 
 // Test optimized CodeUnitAt and array access.
 
 import "package:expect/expect.dart";
 
 
 String one_byte = "hest";
 String two_byte = "h\u{2029}ns";
 
 int testOneByteCodeUnitAt(String x, int j) {
@@ skipping 57 matching lines @@
     Expect.equals(248, testConstantStringCodeUnitAt(1));
     Expect.equals(101, testConstantIndexCodeUnitAt(one_byte));
   }
   for (int j = 0; j < 20; j++) {
     Expect.equals(436, testOneByteCodeUnitAtInLoop(one_byte));
     Expect.equals(8562, testTwoByteCodeUnitAtInLoop(two_byte));
   }
   Expect.throws(() => testOneByteCodeUnitAtInLoop(123));
   Expect.throws(() => testTwoByteCodeUnitAtInLoop(123));
 }