Use zones to capture unexpected errors in barback.

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 16b4090334effbf86296b048b664f5b82f668260..8c30be3d0647a4b89d45e18b1c8674edf8ba0a98 100644
--- a/pkg/barback/lib/src/asset_cascade.dart
+++ b/pkg/barback/lib/src/asset_cascade.dart
@@ -274,17 +274,6 @@ class AssetCascade {
       // TODO(rnystrom): Put some useful data in here.
       _resultsController.add(
           new BuildResult(_accumulatedErrors));
-    }).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
-      // boundary.
-      //
-      // 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, stackTrace);
-    }).whenComplete(() {
       _processDone = null;
       _accumulatedErrors = null;
     });