Ensure that errors have stack traces attached.

pkg/barback/lib/src/asset_cascade.dart

Index: pkg/barback/lib/src/asset_cascade.dart
diff --git a/pkg/barback/lib/src/asset_cascade.dart b/pkg/barback/lib/src/asset_cascade.dart
index 848d0a2caac1c9cfc18bf8be75ec99cf01336d56..df98f1dfcc828673919ae32c021694a4290cfc7c 100644
--- a/pkg/barback/lib/src/asset_cascade.dart
+++ b/pkg/barback/lib/src/asset_cascade.dart
@@ -266,7 +266,7 @@ class AssetCascade {
       // TODO(rnystrom): Put some useful data in here.
       _resultsController.add(
           new BuildResult(_accumulatedErrors));
-    }).catchError((error) {
+    }).catchError((error, stackTrace) {
       // If we get here, it's an unexpected error. Runtime errors like missing
       // assets should be handled earlier. Errors from transformers or other
       // external code that barback calls into should be caught at that API
@@ -275,7 +275,7 @@ class AssetCascade {
       // On the off chance we get here, pipe the error to the results stream
       // as an error. That will let applications handle it without it appearing
       // in the same path as "normal" errors that get reported.
-      _resultsController.addError(error);
+      _resultsController.addError(error, stackTrace);
     }).whenComplete(() {
       _processDone = null;
       _accumulatedErrors = null;