Support dart2js in pub serve.

pkg/barback/lib/src/transform_logger.dart

Index: pkg/barback/lib/src/transform_logger.dart
diff --git a/pkg/barback/lib/src/transform_logger.dart b/pkg/barback/lib/src/transform_logger.dart
index 131be29e8f54d14f39e71378a23c942378ebf443..114529b261997ea3d9b7a678a882cfabdfcd7df5 100644
--- a/pkg/barback/lib/src/transform_logger.dart
+++ b/pkg/barback/lib/src/transform_logger.dart
@@ -14,6 +14,14 @@ class TransformLogger {
 
   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