Add a command line option to use the new CPS based backend.

tests/compiler/dart2js/js_backend_cps_ir_operators_test.dart

 // Copyright (c) 2014, 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=-DUSE_CPS_IR=true
 
 // Tests of operators.
 
 library operators_tests;
 
 import 'js_backend_cps_ir.dart';
 
 const List<TestEntry> tests = const [
   const TestEntry("main() { return true ? 42 : 'foo'; }"),
   const TestEntry("""
@@ skipping 55 matching lines @@
   var list = [1, 2, 3];
   J.getInterceptor$a(list).$indexSet(list, 1, 6);
   P.print(list);
   return null;
 }"""),
 ];
 
 void main() {
   runTests(tests);
 }