Adapt streams for additional stackTrace argument.

pkg/barback/lib/src/package_graph.dart

Index: pkg/barback/lib/src/package_graph.dart
diff --git a/pkg/barback/lib/src/package_graph.dart b/pkg/barback/lib/src/package_graph.dart
index 2cd5b46b370e2eac2d4f8082c8859efcdf0d105e..1eedb3e8d70299fc8cd4cfd07bb36b5a7999ba20 100644
--- a/pkg/barback/lib/src/package_graph.dart
+++ b/pkg/barback/lib/src/package_graph.dart
@@ -83,9 +83,9 @@ class PackageGraph {
         // errors, the result will automatically be considered a success.
         _resultsController.add(new BuildResult(unionAll(
             _cascadeResults.values.map((result) => result.errors))));
-      }, onError: (error) {
+      }, onError: (error, [stackTrace]) {
         _lastUnexpectedError = error;
-        _resultsController.addError(error);
+        _resultsController.addError(error, stackTrace);
       });
     }