Support dart2js in pub serve.

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';
 
 /// Object used to report warnings and errors encountered while running a
 /// transformer.
 class TransformLogger {
 
   bool _shouldPrint;
 
   TransformLogger(this._shouldPrint);
 
+  /// Logs an informative message.
+  ///
+  /// If present, [span] indicates the location in the input asset that caused
+  /// the message.
+  void info(String message, [Span span]) {
+    _printMessage('info', message, span);
+  }
+
   /// Logs a warning message.
   ///
   /// If present, [span] indicates the location in the input asset that caused
   /// the warning.
   void warning(String message, [Span span]) {
     _printMessage('warning', message, span);
   }
 
   /// Logs an error message.
   ///
   /// 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, [Span span]) {
     _printMessage('error', message, span);
   }
 
   // TODO(sigmund,rnystrom): do something better than printing.
   _printMessage(String prefix, String message, Span span) {
     if (!_shouldPrint) return;
     print(span == null ? '$prefix $message'
         : '$prefix ${span.getLocationMessage(message)}');
   }
 }