Support --use-kernel in memory_compiler

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 'compiler_helper.dart';
 
 const String TEST_1 = r"""
   import 'dart:_foreign_helper';
@@ skipping 71 matching lines @@
     // exception.
   }
 """;
 
 main() {
   Future check(String test) {
     var checker = checkerForAbsentPresent(test);
     Uri uri = new Uri(scheme: 'dart', path: 'test');
     var compiler = compilerFor(test, uri, expectedErrors: 0);
     return compiler.run(uri).then((_) {
-      var element = findElement(compiler, 'main');
+      MemberElement element = findElement(compiler, 'main');
       var backend = compiler.backend;
       String generated = backend.getGeneratedCode(element);
       checker(generated);
     });
   }
 
   asyncTest(() => Future.wait([
         check(TEST_1),
         check(TEST_2),
         check(TEST_3),
         check(TEST_4),
         check(TEST_5),
       ]));
 }