Add default values for optional parameters. (#27197)

diff --git a/packages/flutter_tools/test/emulator_test.dart b/packages/flutter_tools/test/emulator_test.dart
index 59b276d..7a7c5b4 100644
--- a/packages/flutter_tools/test/emulator_test.dart
+++ b/packages/flutter_tools/test/emulator_test.dart
@@ -203,8 +203,8 @@
     Map<String, String> environment,
     bool includeParentEnvironment = true,
     bool runInShell = false,
-    Encoding stdoutEncoding,
-    Encoding stderrEncoding
+    Encoding stdoutEncoding = systemEncoding,
+    Encoding stderrEncoding = systemEncoding
   }) {
     final String program = command[0];
     final List<String> args = command.sublist(1);