Remove usage of dart:json.

pkg/http/lib/testing.dart

Index: pkg/http/lib/testing.dart
diff --git a/pkg/http/lib/testing.dart b/pkg/http/lib/testing.dart
index 9fe7b0618460ee4bb40e9508dc04dc7f128b520d..45d2c610a5da85d96b8d60e95775e2015e71f416 100644
--- a/pkg/http/lib/testing.dart
+++ b/pkg/http/lib/testing.dart
@@ -8,14 +8,14 @@
 /// allows test code to set up a local request handler in order to fake a server
 /// that responds to HTTP requests:
 ///
-///     import 'dart:json' as json;
+///     import 'dart:convert';
 ///     import 'package:http/testing.dart';
 ///
 ///     var client = new MockClient((request) {
 ///       if (request.url.path != "/data.json") {
 ///         return new Response("", 404);
 ///       }
-///       return new Response(json.stringify({
+///       return new Response(JSON.encode({
 ///         'numbers': [1, 4, 15, 19, 214]
 ///       }, 200, headers: {
 ///         'content-type': 'application/json'