Address review comments.

Author: a-siva

shell/common/switches.h

@@ -194,7 +194,7 @@ DEF_SWITCH(DisableDartAsserts,
            "with assertions disabled in the debug product mode (i.e. with JIT "
            "or DBC).")
 DEF_SWITCH(EnableSerialGC,
-           "enable_serial_gc",
+           "enable-serial-gc",
            "On low power devices with low core counts, running concurrent "
            "GC tasks on threads can cause them to contend with the UI thread "
            "which could potentially lead to jank. This option turns off all "

testing/dart/serial_gc_test.dart

@@ -1,8 +1,8 @@
-// Copyright 2022 The Flutter Authors. All rights reserved.
+// Copyright 2013 The Flutter Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// FlutterTesterOptions=--serial_gc
+// FlutterTesterOptions=--enable-serial-gc
 
 import 'package:litetest/litetest.dart';