Replace the configuration map with a typed object.

tools/testing/dart/main.dart

Index: tools/testing/dart/main.dart
diff --git a/tools/testing/dart/main.dart b/tools/testing/dart/main.dart
index ff2a0d1264180d49555116e52d204e360e1bbe6a..146bb75dc5a213e726e605022eca734c29706b59 100644
--- a/tools/testing/dart/main.dart
+++ b/tools/testing/dart/main.dart
@@ -34,7 +34,7 @@ void main(List<String> arguments) {
   // 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