Load parallel transformers in the same isolate.

sdk/lib/_internal/pub/asset/dart/transformer_isolate.dart

Index: sdk/lib/_internal/pub/asset/dart/transformer_isolate.dart
diff --git a/sdk/lib/_internal/pub/asset/dart/transformer_isolate.dart b/sdk/lib/_internal/pub/asset/dart/transformer_isolate.dart
index 731f1e104488d890569019da7b17abc0c9b9a43e..a3d6a7131d4b070e2c1ca208af5c42d67c33e1ee 100644
--- a/sdk/lib/_internal/pub/asset/dart/transformer_isolate.dart
+++ b/sdk/lib/_internal/pub/asset/dart/transformer_isolate.dart
@@ -16,7 +16,9 @@ import 'serialize.dart';
 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']);