Remove usage of dart:json.

pkg/source_maps/test/builder_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.source_maps_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('builder - with span', () {
     var map = (new SourceMapBuilder()
         ..addSpan(inputVar1, outputVar1)
         ..addSpan(inputFunction, outputFunction)
         ..addSpan(inputVar2, outputVar2)
         ..addSpan(inputExpr, outputExpr))
         .build(output.url);
     expect(map, equals(EXPECTED_MAP));
   });
 
   test('builder - with location', () {
     var str = (new SourceMapBuilder()
         ..addLocation(inputVar1.start, outputVar1.start, 'longVar1')
         ..addLocation(inputFunction.start, outputFunction.start, 'longName')
         ..addLocation(inputVar2.start, outputVar2.start, 'longVar2')
         ..addLocation(inputExpr.start, outputExpr.start, null))
         .toJson(output.url);
-    expect(str, json.stringify(EXPECTED_MAP));
+    expect(str, JSON.encode(EXPECTED_MAP));
   });
 }