Revert "Remove Compiler.assembledCode."

tests/compiler/dart2js/backend_dart/dart_new_backend_test.dart

 // Copyright (c) 2014, 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 '../compiler_helper.dart';
 import 'dart:async';
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/src/dart2jslib.dart';
 import 'package:compiler/src/dart_backend/dart_backend.dart';
 import 'package:compiler/src/dart_backend/backend_ast_to_frontend_ast.dart'
     show INSERT_NEW_BACKEND_COMMENT;
 
 const String TestStaticField = """
 class Foo {
   static int x = 1;
 }
 main() {
   print(Foo.x);
 }
 """;
 
 Future<String> compile(String code) {
-  OutputCollector outputCollector = new OutputCollector();
   MockCompiler compiler = new MockCompiler.internal(
       emitJavaScript: false,
-      enableMinification: false,
-      outputProvider: outputCollector);
+      enableMinification: false);
   return compiler.init().then((_) {
     compiler.parseScript(code);
     Element element = compiler.mainApp.find('main');
     if (element == null) return null;
     compiler.mainFunction = element;
     compiler.phase = Compiler.PHASE_RESOLVING;
     compiler.backend.enqueueHelpers(compiler.enqueuer.resolution,
                                     compiler.globalDependencies);
     compiler.processQueue(compiler.enqueuer.resolution, element);
     compiler.world.populate();
     compiler.backend.onResolutionComplete();
     compiler.irBuilder.buildNodes();
     compiler.phase = Compiler.PHASE_COMPILING;
     DartBackend backend = compiler.backend;
     backend.assembleProgram();
-    String generated = outputCollector.getOutput('', 'dart');
+    String generated = compiler.assembledCode;
     return generated;
   });
 }
 
 Future test(String code, String feature) {
   return compile(code).then((output) {
     if (output == null || !output.contains('main() /* new backend */')) {
       throw 'New backend appears to be deactivated for $feature.\n'
             'Output was: $output';
     }
   });
 }
 
 main() {
   INSERT_NEW_BACKEND_COMMENT = true;
 
   asyncTest(() => Future.forEach([
     () => test(TestStaticField, 'static fields'),
   ], (f) => f()));
 }