Use memory compiler in compiler_helper.dart.

tests/compiler/dart2js/pretty_parameter_test.dart

 // Copyright (c) 2011, 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 parameters keep their names in the output.
 
 import 'dart:async';
 import 'package:expect/expect.dart';
 import 'package:async_helper/async_helper.dart';
 import 'compiler_helper.dart';
 
@@ skipping 17 matching lines @@
     } else {
       t0 = 3;
     }
     print(t0);
   }
   print(t0);
 }
 """;
 
 const String NO_LOCAL = r"""
-foo(bar, bar) {
+foo(bar, baz) {
   if (bar) {
     baz = 2;
   } else {
     baz = 3;
   }
   return baz;
 }
 """;
 
 const String MULTIPLE_PHIS_ONE_LOCAL = r"""
@@ skipping 48 matching lines @@
     compile(NO_LOCAL, entry: 'foo', check: (String generated) {
       Expect.isFalse(generated.contains('var'));
     }),
 
     compile(PARAMETER_INIT, entry: 'foo', check: (String generated) {
       // Check that there is only one var declaration.
       checkNumberOfMatches(new RegExp("var").allMatches(generated).iterator, 1);
     }),
   ]));
 }