Add stack chain support to pub.

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

Index: sdk/lib/_internal/pub/lib/src/barback.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback.dart b/sdk/lib/_internal/pub/lib/src/barback.dart
index 6f14935ffeb3269707bb93b3664d24e9a8a10697..9fd0e5b17e93ae4e63c991661d9bed8310c994ce 100644
--- a/sdk/lib/_internal/pub/lib/src/barback.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback.dart
@@ -141,7 +141,7 @@ Future<BarbackServer> createServer(String host, int port, PackageGraph graph,
 
   return BarbackServer.bind(host, port, barback, graph.entrypoint.root.name)
       .then((server) {
-    return new Future.sync(() {
+    return syncFuture(() {
       if (watcher != WatcherType.NONE) {
         return watchSources(graph, barback, watcher);
       }
@@ -157,7 +157,7 @@ Future<BarbackServer> createServer(String host, int port, PackageGraph graph,
         server.barback.errors.listen((error) {
           if (error is TransformerException) error = error.error;
           if (!completer.isCompleted) {
-            completer.completeError(error, new Trace.current());
+            completer.completeError(error, new Chain.current());
           }
         }),
         server.barback.results.listen((_) {}, onError: (error, stackTrace) {