Tighten types in test.dart even more.

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 'options.dart';
 import 'test_configurations.dart';
 import 'test_suite.dart';
 
 void main(List<String> arguments) {
   TestUtils.setDartDirUri(Uri.base);
-  var configurations = <Map>[];
+  var configurations = <Map<String, dynamic>>[];
   for (var argument in arguments) {
     configurations.addAll(new OptionsParser().parse(argument.split(" ")));
   }
   testConfigurations(configurations);
 }