Use our own DiagnosticHandler for dart2js.

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.
@@ skipping 24 matching lines @@
   /// 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 $message'
+    print(span == null ? '$prefix: $message'
         : '$prefix ${span.getLocationMessage(message)}');
   }
 }