Remove the "pkgbuild" step on the bots.

tools/testing/dart/package_testing_support.dart

 // Copyright (c) 2017, 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.md file.
 
 library package_testing_support;
 
-import 'dart:convert' show JSON;
-
 import 'test_configurations.dart' show testConfigurations;
 
 import 'test_options.dart' show TestOptionsParser;
 
 import 'test_suite.dart' show TestUtils;
 
 main(List<String> arguments) {
   TestUtils.setDartDirUri(Uri.base);
   List<Map> configurations = <Map>[];
   for (String argument in arguments) {
     configurations.addAll(new TestOptionsParser().parse(argument.split(" ")));
   }
   testConfigurations(configurations);
 }