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

tests/compiler/dart2js/js_backend_cps_ir_control_flow_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 control flow statements.
 
 library control_flow_tests;
 
 import 'js_backend_cps_ir.dart';
 
 const List<TestEntry> tests = const [
   const TestEntry("""
 main() {
@@ skipping 119 matching lines @@
 }""","""
 function() {
   P.identical(V.foo(), true) ? P.print("bad") : P.print("good");
   return null;
 }"""),
 ];
 
 void main() {
   runTests(tests);
 }