apply lint prefer_void_to_null in packages/flutter_tools (#22686)

packages/flutter_tools/test/emulator_test.dart

diff --git a/packages/flutter_tools/test/emulator_test.dart b/packages/flutter_tools/test/emulator_test.dart
index 0bb1499..59b276d 100644
--- a/packages/flutter_tools/test/emulator_test.dart
+++ b/packages/flutter_tools/test/emulator_test.dart
@@ -58,7 +58,7 @@
       final TestEmulatorManager testEmulatorManager =
           TestEmulatorManager(emulators);
 
-      Future<Null> expectEmulator(String id, List<Emulator> expected) async {
+      Future<void> expectEmulator(String id, List<Emulator> expected) async {
         expect(await testEmulatorManager.getEmulatorsMatching(id), expected);
       }