Revert "Remove Compiler.assembledCode."

tests/compiler/dart2js/uri_retention_test.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.
 
 library dart2js.test.uri_retention_test;
 
 import 'dart:async';
 
 import 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
 
 import 'memory_compiler.dart' show
-    compilerFor, OutputCollector;
+    compilerFor;
 
 Future<String> compileSources(sources, {bool minify, bool preserveUri}) {
   var options = [];
   if (minify) options.add("--minify");
   if (preserveUri) options.add("--preserve-uris");
-  OutputCollector outputCollector = new OutputCollector();
-  var compiler = compilerFor(
-      sources, options: options, outputProvider: outputCollector);
+  var compiler = compilerFor(sources, options: options);
   return compiler.runCompiler(Uri.parse('memory:main.dart')).then((_) {
-    return outputCollector.getOutput('', 'js');
+    return compiler.assembledCode;
   });
 }
 
 Future test(sources, { bool libName, bool fileName }) {
   return
       compileSources(sources, minify: false, preserveUri: false).then((output) {
     // Unminified the sources should always contain the library name and the
     // file name.
     Expect.isTrue(output.contains("main_lib"));
     Expect.isTrue(output.contains("main.dart"));
@@ skipping 70 matching lines @@
 library main_lib;
 
 @MirrorsUsed(targets: 'main_lib')
 import 'dart:mirrors';
 
 main() {
   print(currentMirrorSystem().findLibrary(#main_lib).uri);
 }
 """,
 };