Remove usage of dart:json.

pkg/source_maps/lib/builder.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.
 
 /// Contains a builder object useful for creating source maps programatically.
 library source_maps.builder;
 
 // TODO(sigmund): add a builder for multi-section mappings.
 
-import 'dart:json' as json;
 import 'dart:collection';
+import 'dart:convert';
 
 import 'span.dart';
 import 'src/vlq.dart';
 
 /// Builds a source map given a set of mappings.
 class SourceMapBuilder {
 
   final List<Entry> _entries = <Entry>[];
 
   /// Indices associated with file urls that will be part of the source map. We
@@ skipping 79 matching lines @@
       'names' : _names.keys.toList(),
       'mappings' : buff.toString()
     };
     if (fileUrl != null) {
       result['file'] = fileUrl;
     }
     return result;
   }
 
   /// Encodes all mappings added to this builder as a json string.
-  String toJson(String fileUrl) => json.stringify(build(fileUrl));
+  String toJson(String fileUrl) => JSON.encode(build(fileUrl));
 
   /// Get the index of [value] in [map], or create one if it doesn't exist.
   int _indexOf(Map<String, int> map, String value) {
     return map.putIfAbsent(value, () {
       int index = map.length;
       map[value] = index;
       return index;
     });
   }
 
@@ skipping 23 matching lines @@
   /// because source map files are encoded by printing each mapping in order as
   /// they appear in the target file.
   int compareTo(Entry other) {
     int res = target.compareTo(other.target);
     if (res != 0) return res;
     res = source.sourceUrl.compareTo(other.source.sourceUrl);
     if (res != 0) return res;
     return source.compareTo(other.source);
   }
 }