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

tests/compiler/dart2js/js_backend_cps_ir_closures_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 closures.
 
 library closures_test;
 
 import 'js_backend_cps_ir.dart';
 
 const List<TestEntry> tests = const [
   const TestEntry("""
 main(x) {
@@ skipping 98 matching lines @@
 """,
 r"""
 function() {
   return new V.A_b_closure(this);
 }"""),
 ];
 
 void main() {
   runTests(tests);
 }