Add default values for optional parameters. (#27197)

packages/flutter_tools/test/base/build_test.dart

diff --git a/packages/flutter_tools/test/base/build_test.dart b/packages/flutter_tools/test/base/build_test.dart
index f2f79fc..ca110fa 100644
--- a/packages/flutter_tools/test/base/build_test.dart
+++ b/packages/flutter_tools/test/base/build_test.dart
@@ -55,7 +55,7 @@
     String packagesPath,
     String depfilePath,
     IOSArch iosArch,
-    Iterable<String> additionalArgs,
+    Iterable<String> additionalArgs = const <String>[],
   }) async {
     _callCount += 1;
     _snapshotType = snapshotType;