Consider errors logged by a transformer equivalent to thrown errors.

pkg/barback/lib/src/base_transform.dart

 // Copyright (c) 2014, 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.base_transform;
 
 import 'dart:async';
 import 'dart:convert';
 
 import 'asset.dart';
@@ skipping 11 matching lines @@
 /// This class provides the transformers with inputs, but its up to the
 /// subclasses to provide a means of emitting outputs.
 abstract class BaseTransform {
   final TransformNode _node;
 
   /// Whether the primary input should be consumed.
   ///
   /// This is exposed via [BaseTransformController].
   bool _consumePrimary = false;
 
+  /// Whether the transformer logged an error.
+  ///
+  /// This is exposed via [BaseTransformController].
+  bool _loggedError = false;
+
   /// The controller for the stream of log entries emitted by the transformer.
   ///
   /// This is exposed via [BaseTransformController].
   ///
   /// This is synchronous because error logs can cause the transform to fail, so
   /// we need to ensure that their processing isn't delayed until after the
   /// transform or build has finished.
   final _onLogController = new StreamController<LogEntry>.broadcast(sync: true);
 
   /// A logger so that the [Transformer] can report build details.
@@ skipping 18 matching lines @@
   Asset get primaryInput {
     if (_node.primary.state != AssetState.AVAILABLE) {
       throw new AssetNotFoundException(_node.primary.id);
     }
 
     return _node.primary.asset;
   }
 
   BaseTransform(this._node) {
     _logger = new TransformLogger((asset, level, message, span) {
+      if (level == LogLevel.ERROR) _loggedError = true;
+
       // If the log isn't already associated with an asset, use the primary.
       if (asset == null) asset = _node.primary.id;
       var entry = new LogEntry(_node.info, asset, level, message, span);
       _onLogController.add(entry);
     });
   }
 
   /// Gets the asset for an input [id].
   ///
   /// If an input with that ID cannot be found, throws an
@@ skipping 36 matching lines @@
 ///
 /// Controllers are used so that [TransformNode]s can get values from a
 /// [BaseTransform] without exposing getters in the public API.
 abstract class BaseTransformController {
   /// The [BaseTransform] controlled by this controller.
   final BaseTransform transform;
 
   /// Whether the primary input should be consumed.
   bool get consumePrimary => transform._consumePrimary;
 
+  /// Whether the transform logged an error.
+  bool get loggedError => transform._loggedError;
+
   /// The stream of log entries emitted by the transformer during a run.
   Stream<LogEntry> get onLog => transform._onLogController.stream;
 
   BaseTransformController(this.transform);
 
   /// Notifies the [BaseTransform] that the transformation has finished being
   /// applied.
   ///
   /// This will close any streams and release any resources that were allocated
   /// for the duration of the transformation.
   void close() {
     transform._onLogController.close();
   }
 }