Add interfaces for a new compiler API.

tests/compiler/dart2js/codegen_helper.dart

 // Copyright (c) 2013, 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:expect/expect.dart';
-
+import 'package:compiler/src/null_compiler_output.dart';
 import 'memory_source_file_helper.dart';
 
 Future<Map<String, String>> generate(String code,
     [List<String> options = const []]) {
   Uri script = currentDirectory.resolveUri(Platform.script);
   Uri libraryRoot = script.resolve('../../../sdk/');
   Uri packageRoot = script.resolve('./packages/');
 
   var provider = new MemorySourceFileProvider({ 'main.dart': code });
   var handler = new FormattingDiagnosticHandler(provider);
 
-  Compiler compiler = new Compiler(provider.readStringFromUri,
-                                   null,
-                                   handler.diagnosticHandler,
+  Compiler compiler = new Compiler(provider,
+                                   const NullCompilerOutput(),
+                                   handler,
                                    libraryRoot,
                                    packageRoot,
                                    options,
                                    {});
   Uri uri = Uri.parse('memory:main.dart');
   return compiler.run(uri).then((success) {
     Expect.isTrue(success);
     Map<String, String> result = new Map<String, String>();
     var backend = compiler.backend;
     for (var element in backend.generatedCode.keys) {
       if (element.compilationUnit.script.readableUri != uri) continue;
       var name = element.name;
       var code = backend.assembleCode(element);
       result[name] = code;
     }
     return result;
   });
 }