Support --use-kernel in memory_compiler

tests/compiler/dart2js/compiler_helper.dart

 // Copyright (c) 2012, 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.
 
 library compiler_helper;
 
 import 'dart:async';
 import "package:expect/expect.dart";
 
 import 'package:compiler/compiler_new.dart';
@@ skipping 113 matching lines @@
     }
 
     CompilationResult result = await runCompiler(
         memorySourceFiles: source,
         options: options,
         outputProvider: outputCollector);
     Expect.isTrue(result.isSuccess);
     Compiler compiler = result.compiler;
     LibraryElement mainApp =
         compiler.frontendStrategy.elementEnvironment.mainLibrary;
-    Element element = mainApp.find(entry);
+    MemberElement element = mainApp.find(entry);
     js.JavaScriptBackend backend = compiler.backend;
     String generated = backend.getGeneratedCode(element);
     if (check != null) {
       check(generated);
     }
     return returnAll ? outputCollector.getOutput('', OutputType.js) : generated;
   }
 }
 
 Future<String> compileAll(String code,
@@ skipping 156 matching lines @@
       }
     }
   }
 
   return checker;
 }
 
 RegExp _directivePattern = new RegExp(
     //      \1                     \2        \3
     r'''// *(present|absent): *(?:"([^"]*)"|'([^'']*)')''', multiLine: true);