Use our own DiagnosticHandler for dart2js.

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 114529b261997ea3d9b7a678a882cfabdfcd7df5..e68573a476e041edb83e1f44e6ce66df670b8510 100644
--- a/pkg/barback/lib/src/transform_logger.dart
+++ b/pkg/barback/lib/src/transform_logger.dart
@@ -42,7 +42,7 @@ class TransformLogger {
   // TODO(sigmund,rnystrom): do something better than printing.
   _printMessage(String prefix, String message, Span span) {
     if (!_shouldPrint) return;
-    print(span == null ? '$prefix $message'
+    print(span == null ? '$prefix: $message'
         : '$prefix ${span.getLocationMessage(message)}');
   }
 }