Work around issue 14971 in barback.

pkg/barback/lib/src/serialize.dart

Index: pkg/barback/lib/src/serialize.dart
diff --git a/pkg/barback/lib/src/serialize.dart b/pkg/barback/lib/src/serialize.dart
index f146f23e885e6c4d08c0b9d2174924b4cb501519..eb948a451fe7ad68c1a5d551fa1138f16998dd59 100644
--- a/pkg/barback/lib/src/serialize.dart
+++ b/pkg/barback/lib/src/serialize.dart
@@ -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));
   });
 }