Update format for latest v3 SDK (#340)

Author: kevmoo

pkgs/shelf/test/message_change_test.dart

@@ -47,11 +47,10 @@ void main() {
 /// the behavior of `change` with different `headers` and `context` values.
 void _testChange(
     Message Function({
-  dynamic body,
-  Map<String, String> headers,
-  Map<String, Object> context,
-})
-        factory) {
+      dynamic body,
+      Map<String, String> headers,
+      Map<String, Object> context,
+    }) factory) {
   group('body', () {
     test('with String', () async {
       var request = factory(body: 'Hello, world');