Fix the error message widget

lib/runtime/messages_widget.dart

 // Copyright (c) 2015, 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.
 
 /// This library displays [MessageSummary]s from the Dart Dev Compiler.
 library dev_compiler.messages;
 
 import 'dart:convert';
 import 'dart:html';
 
@@ skipping 38 matching lines @@
     for (var m in messages) {
       var message = _hyperlinkUrls(_escape(m.message));
       var span = m.span;
       var sb = new StringBuffer();
       sb.write('<div class="message"><div class="text $level">$message</div>');
       if (span != null) {
         sb.write('<div class="location">'
             '  <span class="location">${span.start.toolString}</span></div>'
             '  <span class="text">');
         if (span is SourceSpanWithContext) {
-          var context = span.context;
-          sb.write(_escape(context.substring(0, span.start.column)));
+          sb.write(_escape(span.context.substring(0, span.start.column)));
           sb.write('<span class="$level">');
           sb.write(_escape(span.text));
           sb.write('</span>');
-          sb.write(_escape(context.substring(span.end.column)));
+          var end = span.start.column + span.text.length;
+          sb.write(_escape(span.context.substring(end)));
         } else {
           sb.write(_escape(span.text));
         }
         sb.write('</span></div></div>');
       }
       sb.write('</div>');
 
       var logElement = new Element.html('$sb',
           validator: new NodeValidatorBuilder.common()
             ..allowNavigation(new _OpenUriPolicy()));
@@ skipping 46 matching lines @@
 class _Visitor extends RecursiveSummaryVisitor {
   final Map<String, List<MessageSummary>> messagesByLevel = {};
 
   @override
   void visitMessage(MessageSummary message) {
     var level = message.level.toLowerCase();
     messagesByLevel.putIfAbsent(level, () => []);
     messagesByLevel[level].add(message);
   }
 }