Add support for lazy transformers.

pkg/barback/lib/src/transform_logger.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_logger;
 
 import 'package:source_maps/span.dart';
 
 import 'asset_id.dart';
 import 'log.dart';
-import 'transform.dart';
+
+typedef void LogFunction(AssetId asset, LogLevel level, String message,
+                         Span span);
 
 /// Object used to report warnings and errors encountered while running a
 /// transformer.
 class TransformLogger {
   final LogFunction _logFunction;
 
   TransformLogger(this._logFunction);
 
   /// Logs an informative message.
   ///
@@ skipping 19 matching lines @@
   ///
   /// If [asset] is provided, the log entry is associated with that asset.
   /// Otherwise it's associated with the primary input of [transformer].
   /// If present, [span] indicates the location in the input asset that caused
   /// the error.
   // TODO(sigmund,nweiz): clarify when an error should be logged or thrown.
   void error(String message, {AssetId asset, Span span}) {
     _logFunction(asset, LogLevel.ERROR, message, span);
   }
 }