Re-apply "Merge pull request dart-archive/json_rpc_2#9 from srawlins/remove-unused-imports"

This reverts commit 16f0cbe and
re-applies commit e0459c8.

It was determined that the build failure was caused by an upgraded SDK
version.

Author: nex3

pkgs/json_rpc_2/test/client/client_test.dart

@@ -3,7 +3,6 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
-import 'dart:convert';
 
 import 'package:test/test.dart';
 import 'package:json_rpc_2/error_code.dart' as error_code;

pkgs/json_rpc_2/test/client/utils.dart

@@ -9,7 +9,6 @@ import 'package:stream_channel/stream_channel.dart';
 import 'package:test/test.dart';
 
 import 'package:json_rpc_2/json_rpc_2.dart' as json_rpc;
-import 'package:json_rpc_2/error_code.dart' as error_code;
 
 /// A controller used to test a [json_rpc.Client].
 class ClientController {

pkgs/json_rpc_2/test/server/batch_test.dart

@@ -2,11 +2,8 @@
 // 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.
 
-import 'dart:convert';
-
 import 'package:test/test.dart';
 import 'package:json_rpc_2/error_code.dart' as error_code;
-import 'package:json_rpc_2/json_rpc_2.dart' as json_rpc;
 
 import 'utils.dart';
 
@@ -89,4 +86,4 @@ void main() {
       }
     }])));
   });
-}
+}

pkgs/json_rpc_2/test/server/invalid_request_test.dart

@@ -2,11 +2,8 @@
 // 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.
 
-import 'dart:convert';
-
 import 'package:test/test.dart';
 import 'package:json_rpc_2/error_code.dart' as error_code;
-import 'package:json_rpc_2/json_rpc_2.dart' as json_rpc;
 
 import 'utils.dart';