Load parallel transformers in the same isolate.

sdk/lib/_internal/pub/asset/dart/transformer_isolate.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.
 
 library pub.asset.transformer_isolate;
 
 import 'dart:convert';
 import 'dart:isolate';
 import 'dart:mirrors';
 
 import 'package:barback/barback.dart';
 
 import 'serialize.dart';
 
 /// Sets up the initial communication with the host isolate.
 void loadTransformers(SendPort replyTo) {
   var port = new ReceivePort();
   replyTo.send(port.sendPort);
-  port.first.then((wrappedMessage) {
+  port.listen((wrappedMessage) {
+    // TODO(nweiz): When issue 19228 is fixed, spin up a separate isolate for
+    // libraries loaded beyond the first so they can run in parallel.
     respond(wrappedMessage, (message) {
       var library = Uri.parse(message['library']);
       var configuration = JSON.decode(message['configuration']);
       var mode = new BarbackMode(message['mode']);
       return _initialize(library, configuration, mode).
           map(serializeTransformerLike).toList();
     });
   });
 }
 
@@ skipping 62 matching lines @@
 
 // Older barbacks don't support [AggregateTransformer], and calling
 // [reflectClass] on an undefined class will throw an error, so we just define a
 // null getter for them.
 //# if barback >=0.14.1
 ClassMirror get _aggregateTransformerClass =>
     reflectClass(AggregateTransformer);
 //# else
 //>   ClassMirror get _aggregateTransformerClass => null;
 //# end