diff --git a/flutter_frontend_server/bin/starter.dart b/flutter_frontend_server/bin/starter.dart
index f78c3a2..8810136 100644
--- a/flutter_frontend_server/bin/starter.dart
+++ b/flutter_frontend_server/bin/starter.dart
@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.8
 library frontend_server;
 
 import 'dart:io';
diff --git a/flutter_frontend_server/lib/server.dart b/flutter_frontend_server/lib/server.dart
index 49f8ffd..5b6543f 100644
--- a/flutter_frontend_server/lib/server.dart
+++ b/flutter_frontend_server/lib/server.dart
@@ -4,6 +4,7 @@
 
 // ignore_for_file: avoid_print
 
+// @dart=2.8
 library flutter_frontend_server;
 
 import 'dart:async';
@@ -25,9 +26,9 @@
 /// version for testing.
 Future<int> starter(
   List<String> args, {
-  frontend.CompilerInterface? compiler,
-  Stream<List<int>>? input,
-  StringSink? output,
+  frontend.CompilerInterface compiler,
+  Stream<List<int>> input,
+  StringSink output,
 }) async {
   ArgResults options;
   try {
diff --git a/flutter_frontend_server/pubspec.yaml b/flutter_frontend_server/pubspec.yaml
index b4cbec7..fc80828 100644
--- a/flutter_frontend_server/pubspec.yaml
+++ b/flutter_frontend_server/pubspec.yaml
@@ -18,7 +18,7 @@
 # relative to this directory into //third_party/dart
 
 environment:
-  sdk: ">=2.12.0 <3.0.0"
+  sdk: ">=2.2.2 <3.0.0"
 
 dependencies:
   args: any