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

tests/compiler/dart2js/js_backend_cps_ir.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.
 
 // Test that the CPS IR code generator compiles programs and produces the
 // the expected output.
 
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 import 'package:compiler/src/apiimpl.dart'
@@ skipping 40 matching lines @@
 
   if (foundElement == null) {
     Expect.fail('There is no compiled element called $name');
   }
 
   js.Node ast = compiler.enqueuer.codegen.generatedCode[foundElement];
   return js.prettyPrint(ast, compiler).getText();
 }
 
 runTests(List<TestEntry> tests) {
-  Expect.isTrue(const bool.fromEnvironment("USE_CPS_IR"),
-      'Run with USE_CPS_IR=true');
-
   for (TestEntry test in tests) {
     Map files = {TEST_MAIN_FILE: test.source};
     asyncTest(() {
-      Compiler compiler = compilerFor(files);
+      Compiler compiler = compilerFor(files, options: <String>['--use-cps-ir']);
       Uri uri = Uri.parse('memory:$TEST_MAIN_FILE');
       return compiler.run(uri).then((bool success) {
         Expect.isTrue(success);
         String expectation = test.expectation;
         if (expectation != null) {
           String expected = test.expectation;
           String found = test.elementName == null
               ? getCodeForMain(compiler)
               : getCodeForMethod(compiler, test.elementName);
           if (expected != found) {
             Expect.fail('Expected:\n$expected\nbut found\n$found');
           }
         }
       }).catchError((e) {
         print(e);
         Expect.fail('The following test failed to compile:\n'
                     '${formatTest(files)}');
       });
     });
   }
 }