Run pub tests against older versions of barback.

third_party/pkg/barback-0.13.0/lib/src/serialize.dart

Index: third_party/pkg/barback-0.13.0/lib/src/serialize.dart
diff --git a/pkg/barback/lib/src/serialize.dart b/third_party/pkg/barback-0.13.0/lib/src/serialize.dart
similarity index 96%
copy from pkg/barback/lib/src/serialize.dart
copy to third_party/pkg/barback-0.13.0/lib/src/serialize.dart
index 145846396c8da330bc4c3a3701cd48a03e2edddd..7114039e44d5cc417baf8dfc62a97400619447c3 100644
--- a/pkg/barback/lib/src/serialize.dart
+++ b/third_party/pkg/barback-0.13.0/lib/src/serialize.dart
@@ -9,7 +9,7 @@ import 'dart:isolate';
 
 import 'package:stack_trace/stack_trace.dart';
 
-import 'asset/asset_id.dart';
+import 'asset_id.dart';
 import 'utils.dart';
 
 /// Converts [id] into a serializable map.
@@ -45,8 +45,10 @@ Stream deserializeStream(SendPort sendPort) {
   return callbackStream(() {
     var receivePort = new ReceivePort();
     sendPort.send(receivePort.sendPort);
+    // TODO(nweiz): use a const constructor for StreamTransformer when issue
+    // 14971 is fixed.
     return receivePort.transform(
-        const StreamTransformer(_deserializeTransformer));
+        new StreamTransformer(_deserializeTransformer));
   });
 }