Use zones to capture unexpected errors in barback.

pkg/barback/lib/src/phase_input.dart

Index: pkg/barback/lib/src/phase_input.dart
diff --git a/pkg/barback/lib/src/phase_input.dart b/pkg/barback/lib/src/phase_input.dart
index 3506560da0ef4cc5898357ca8323c5eac93591f9..d89aedaab6fc8454b42c6a0c6fe7cbb5faf17c50 100644
--- a/pkg/barback/lib/src/phase_input.dart
+++ b/pkg/barback/lib/src/phase_input.dart
@@ -193,10 +193,6 @@ class PhaseInput {
     }).whenComplete(() {
       _adjustTransformersFuture = null;
     });
-
-    // Don't top-level errors coming from the input processing. Any errors will
-    // eventually be piped through [process]'s returned Future.
-    _adjustTransformersFuture.catchError((_) {});
   }
 
   // Remove any old transforms that used to have [asset] as a primary asset but