Use zones to capture unexpected errors in barback.

pkg/barback/lib/src/asset_cascade.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.asset_cascade;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'asset.dart';
@@ skipping 256 matching lines @@
   Future _waitForProcess() {
     if (_processDone != null) return _processDone;
 
     _accumulatedErrors = new Queue();
     _numLogErrors = 0;
     return _processDone = _process().then((_) {
       // Report the build completion.
       // 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;
     });
   }
 
   /// Starts the background processing.
   ///
   /// Returns a future that completes when all assets have been processed.
   Future _process() {
     _newChanges = false;
@@ skipping 14 matching lines @@
         return null;
       }
 
       // Process that phase and then loop onto the next.
       return future.then((_) => _process());
     });
   }
 
   String toString() => "cascade for $package";
 }