Get rid of Chain.track in pub.

sdk/lib/_internal/pub/lib/src/barback/barback_server.dart

Index: sdk/lib/_internal/pub/lib/src/barback/barback_server.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/barback_server.dart b/sdk/lib/_internal/pub/lib/src/barback/barback_server.dart
index 7d9125a89fa446bd6721815c53467fb7862b5553..2f1b4f8e350f0d93c9dfad1c2448a0d7ea5dc07e 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/barback_server.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/barback_server.dart
@@ -54,7 +54,7 @@ class BarbackServer extends BaseServer<BarbackServerResult> {
   static Future<BarbackServer> bind(AssetEnvironment environment,
       String host, int port, {String package, String rootDirectory}) {
     if (package == null) package = environment.rootPackage.name;
-    return Chain.track(bindServer(host, port)).then((server) {
+    return bindServer(host, port).then((server) {
       if (rootDirectory == null) {
         log.fine('Serving packages on $host:$port.');
       } else {