Replace the configuration map with a typed object.

tools/testing/dart/main.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 file.
 
 /// This file is the entrypoint of the Dart repository's custom test system.
 /// It is used to test:
 ///
 ///     1. The Dart VM
 ///     2. The dart2js compiler
 ///     3. The static analyzer
@@ skipping 16 matching lines @@
 import "test_suite.dart";
 
 /// Runs all of the tests specified by the given command line [arguments].
 void main(List<String> arguments) {
   // This script is in "<repo>/tools/testing/dart".
   TestUtils.setDartDirUri(Platform.script.resolve('../../..'));
 
   // Parse the command line arguments to a configuration.
   var parser = new OptionsParser();
   var configurations = parser.parse(arguments);
-  if (configurations == null || configurations.isEmpty) return;
+  if (configurations.isEmpty) return;
 
   // Run all of the configured tests.
   // TODO(26372): Ensure that all tasks complete and return a future from this
   // function.
   testConfigurations(configurations);
 }