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

tests/language/constant_propagation_phis_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
+// VMOptions=--optimization-counter-threshold=10 --no-background-compilation
 
 import "package:expect/expect.dart";
 
 // Test that constant propagation correctly updates phis when predecessor's
 // reachability changes.
 
 final keys = const ["keyA"];
 final values = const ["a"];
 
 main() {
@@ skipping 10 matching lines @@
   return (index == -1) ? null : values[index];
 }
 
 indexOf(keys, key) {
   for (var i = keys.length - 1; i >= 0; i--) {
     var equals = keys[i] == key;
     if (equals) return i;
   }
   return -1;
 }