Make source map location information more readable.

pkg/csslib/lib/src/messages.dart

 // Copyright (c) 2012, 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 csslib.src.messages;
 
 import 'package:logging/logging.dart' show Level;
 import 'package:source_maps/span.dart' show Span;
 
 import 'options.dart';
@@ skipping 42 matching lines @@
     var output = new StringBuffer();
     bool colors = useColors && _ERROR_COLORS.containsKey(level);
     var levelColor =  _ERROR_COLORS[level];
     if (colors) output.write(levelColor);
     output..write(_ERROR_LABEL[level])..write(' ');
     if (colors) output.write(NO_COLOR);
 
     if (span == null) {
       output.write(message);
     } else {
+      output.write('on ');
       output.write(span.getLocationMessage(message, useColors: colors,
           color: levelColor));
     }
 
     return output.toString();
   }
 }
 
 typedef void PrintHandler(Message obj);
 
@@ skipping 45 matching lines @@
   }
 
   /** Merge [newMessages] to this message lsit. */
   void mergeMessages(Messages newMessages) {
     messages.addAll(newMessages.messages);
     newMessages.messages.where((message) =>
         message.level.value == Level.SEVERE || options.verbose)
         .forEach((message) { printHandler(message); });
   }
 }