html escape log messages

pkg/polymer/lib/src/build/wrapped_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 polymer.src.build.wrapped_logger;
 
 import 'dart:async';
 import 'dart:convert';
 
 import 'package:barback/barback.dart';
@@ skipping 69 matching lines @@
           transform.addOutput(new Asset.fromString(
               primaryInputId.addExtension(common.LOG_EXTENSION),
               JSON.encode(logs)));
         });
   }
 
   void _addLog(AssetId assetId, LogLevel level, String message,
                SourceSpan span) {
     var data = {
         'level': level.name,
-        'message': message,
+        'message': const HtmlEscape().convert(message),
     };
     if (assetId != null) {
       data['assetId'] = {
           'package': assetId.package,
           'path': assetId.path,
       };
     }
     if (span != null) {
       data['span'] = {
           'location': span.start.toolString,
           'text': new HtmlEscape().convert(span.text),
       };
     }
     _logs.add(data);
   }
-}
+}