Use memory compiler in compiler_helper.dart.

tests/compiler/dart2js/js_spec_optimization_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.
 
 import 'dart:async';
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 import 'compiler_helper.dart';
 
 const String TEST_1 = r"""
@@ skipping 52 matching lines @@
       //      \1                    \2        \3
       r'''// *(present|absent): (?:"([^"]*)"|'([^'']*)')''',
       multiLine: true);
 
   Future check(String test) {
     Uri uri = new Uri(scheme: 'dart', path: 'test');
     var compiler = compilerFor(test, uri, expectedErrors: 0);
     return compiler.runCompiler(uri).then((_) {
       var element = findElement(compiler, 'main');
       var backend = compiler.backend;
-      String generated = backend.assembleCode(element);
+      String generated = backend.getGeneratedCode(element);
 
       for (Match match in directivePattern.allMatches(test)) {
         String directive = match.group(1);
         String pattern = match.groups([2, 3]).where((s) => s != null).single;
         if (directive == 'present') {
           Expect.isTrue(generated.contains(pattern),
               "Cannot find '$pattern' in:\n$generated");
         } else {
           assert(directive == 'absent');
           Expect.isFalse(generated.contains(pattern),
               "Must not find '$pattern' in:\n$generated");
         }
       }
     });
   }
 
   asyncTest(() => Future.wait([
     check(TEST_1),
     check(TEST_2),
     check(TEST_3),
   ]));
 }