Remove usage of dart:json.

pkg/source_maps/test/parser_test.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 test.parser_test;
 
-import 'dart:json' as json;
+import 'dart:convert';
 import 'package:unittest/unittest.dart';
 import 'package:source_maps/source_maps.dart';
 import 'common.dart';
 
 main() {
   test('parse', () {
     var mapping = parseJson(EXPECTED_MAP);
     check(outputVar1, mapping, inputVar1, false);
     check(outputVar2, mapping, inputVar2, false);
     check(outputFunction, mapping, inputFunction, false);
     check(outputExpr, mapping, inputExpr, false);
   });
 
   test('parse + json', () {
-    var mapping = parse(json.stringify(EXPECTED_MAP));
+    var mapping = parse(JSON.encode(EXPECTED_MAP));
     check(outputVar1, mapping, inputVar1, false);
     check(outputVar2, mapping, inputVar2, false);
     check(outputFunction, mapping, inputFunction, false);
     check(outputExpr, mapping, inputExpr, false);
   });
 
   test('parse with file', () {
     var mapping = parseJson(EXPECTED_MAP);
     check(outputVar1, mapping, inputVar1, true);
     check(outputVar2, mapping, inputVar2, true);
     check(outputFunction, mapping, inputFunction, true);
     check(outputExpr, mapping, inputExpr, true);
   });
 }