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

tests/compiler/dart2js/js_backend_cps_ir_constructor_test.dart

 // Copyright (c) 2015, 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 interceptors.
 
 library constructor_test;
 
 import 'js_backend_cps_ir.dart';
 
 const List<TestEntry> tests = const [
   const TestEntry.forMethod('generative_constructor(Sub#)', """
 class Base {
@@ skipping 250 matching lines @@
 }""", r"""
 function() {
   V.B$(5, P.$int).get$typevar();
   return null;
 }"""),
 ];
 
 void main() {
   runTests(tests);
 }