Support passing configuration data to a transformer plugin.

sdk/lib/_internal/pub/test/serve/utils.dart

Index: sdk/lib/_internal/pub/test/serve/utils.dart
diff --git a/sdk/lib/_internal/pub/test/serve/utils.dart b/sdk/lib/_internal/pub/test/serve/utils.dart
index 7025d9b96e4dab37c6097efb0bd04330e75c7105..46ace9de09ed1eb745219c360397cb2feee27184 100644
--- a/sdk/lib/_internal/pub/test/serve/utils.dart
+++ b/sdk/lib/_internal/pub/test/serve/utils.dart
@@ -26,7 +26,7 @@ import 'dart:async';
 import 'package:barback/barback.dart';
 
 class RewriteTransformer extends Transformer {
-  RewriteTransformer();
+  RewriteTransformer.asPlugin();
 
   String get allowedExtensions => '.txt';
 
@@ -69,7 +69,7 @@ const TOKEN = "$id";
 final _tokenRegExp = new RegExp(r'^const TOKEN = "(.*?)";\$', multiLine: true);
 
 class DartTransformer extends Transformer {
-  DartTransformer();
+  DartTransformer.asPlugin();
 
   String get allowedExtensions => '.dart';