Show implicit pub dependency on barback in errors.

sdk/lib/_internal/pub/test/test_pub.dart

Index: sdk/lib/_internal/pub/test/test_pub.dart
diff --git a/sdk/lib/_internal/pub/test/test_pub.dart b/sdk/lib/_internal/pub/test/test_pub.dart
index 71ce02c8a3bb70e938aeb59eb2036a322f0f7786..df1bf1b2b24fcdf93cd792445fbaa840e1864d98 100644
--- a/sdk/lib/_internal/pub/test/test_pub.dart
+++ b/sdk/lib/_internal/pub/test/test_pub.dart
@@ -147,6 +147,7 @@ Future _closeServer() {
   if (_server == null) return new Future.value();
   var future = _server.close();
   _server = null;
+  _hasServer = false;
   _portCompleterCache = null;
   return future;
 }