Remove usage of dart:json.

pkg/polymer/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 messages;
 
-import 'dart:json' as json;
+import 'dart:convert';
 
 import 'package:barback/barback.dart' show TransformLogger;
 import 'package:source_maps/span.dart' show Span;
 import 'package:logging/logging.dart' show Level;
 
 import 'compiler_options.dart';
 import 'utils.dart';
 
 /** Map between error levels and their display color. */
 final Map<Level, String> _ERROR_COLORS = (() {
@@ skipping 29 matching lines @@
     } else {
       output.write(span.getLocationMessage(message, useColors: colors,
           color: levelColor));
     }
 
     return output.toString();
   }
 
   String toJson() {
     if (span == null) return toString();
-    return json.stringify([{
+    return JSON.encode([{
       'method': kind,
       'params': {
         'file': span.sourceUrl,
         'message': message,
         'line': span.start.line + 1,
         'charStart': span.start.offset,
         'charEnd': span.end.offset,
       }
     }]);
   }
@@ skipping 72 matching lines @@
         useColors: options.useColors);
 
     messages.add(msg);
     if (options.verbose) printMessage(msg);
   }
 
   void printMessage(msg) {
     if (shouldPrint) print(options.jsonFormat ? msg.toJson() : msg);
   }
 }