Make barback propagate logs synchronously.

pkg/barback/lib/src/transform_node.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.transform_node;
 
 import 'dart:async';
 
 import 'asset.dart';
 import 'asset_id.dart';
@@ skipping 37 matching lines @@
   /// A stream that emits an event whenever this transform becomes dirty and
   /// needs to be re-run.
   ///
   /// This may emit events when the transform was already dirty or while
   /// processing transforms. Events are emitted synchronously to ensure that the
   /// dirty state is thoroughly propagated as soon as any assets are changed.
   Stream get onDirty => _onDirtyController.stream;
   final _onDirtyController = new StreamController.broadcast(sync: true);
 
   /// A stream that emits an event whenever this transform logs an entry.
+  ///
+  /// 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.
   Stream<LogEntry> get onLog => _onLogController.stream;
-  final _onLogController = new StreamController<LogEntry>.broadcast();
+  final _onLogController = new StreamController<LogEntry>.broadcast(sync: true);
 
   TransformNode(this.phase, this.transformer, this.primary) {
     _primarySubscription = primary.onStateChange.listen((state) {
       if (state.isRemoved) {
         remove();
       } else {
         _dirty();
       }
     });
   }
@@ skipping 137 matching lines @@
   }
 
   void _log(AssetId asset, LogLevel level, String message, Span span) {
     // If the log isn't already associated with an asset, use the primary.
     if (asset == null) asset = primary.id;
     var info = new TransformInfo(transformer, primary.id);
     var entry = new LogEntry(info, asset, level, message, span);
     _onLogController.add(entry);
   }
 }