Use zones to capture unexpected errors in barback.

pkg/barback/lib/src/phase_input.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library barback.phase_input;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'asset.dart';
@@ skipping 175 matching lines @@
       if (error is! AssetNotFoundException || error.id != input.id) {
         throw error;
       }
 
       // If the asset is removed, [_tryUntilStable] will throw an
       // [AssetNotFoundException]. In that case, just remove it.
       remove();
     }).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
   // no longer apply to its new contents.
   Future _removeStaleTransforms(Asset asset, Set<Transformer> transformers) {
     return Future.wait(_transforms.map((transform) {
       return newFuture(() {
         if (!transformers.contains(transform.transformer)) return false;
 
         // TODO(rnystrom): Catch all errors from isPrimary() and redirect to
@@ skipping 111 matching lines @@
     }
 
     return Future.wait(_transforms.map((transform) {
       if (!transform.isDirty) return new Future.value(new Set());
       return transform.apply();
     })).then((outputs) => unionAll(outputs));
   }
 
   String toString() => "phase input in $_location for $input";
 }