feat: migrate base/dds.dart to null-safety (#88015)

packages/flutter_tools/lib/src/base/dds.dart

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'dart:async';
 
 import 'package:dds/dds.dart' as dds;
@@ -24,10 +22,10 @@ Future<dds.DartDevelopmentService> Function(
 /// Helper class to launch a [dds.DartDevelopmentService]. Allows for us to
 /// mock out this functionality for testing purposes.
 class DartDevelopmentService {
-  dds.DartDevelopmentService _ddsInstance;
+  dds.DartDevelopmentService? _ddsInstance;
 
-  Uri get uri => _ddsInstance?.uri ?? _existingDdsUri;
-  Uri _existingDdsUri;
+  Uri? get uri => _ddsInstance?.uri ?? _existingDdsUri;
+  Uri? _existingDdsUri;
 
   Future<void> get done => _completer.future;
   final Completer<void> _completer = Completer<void>();
@@ -37,7 +35,7 @@ class DartDevelopmentService {
     int hostPort,
     bool ipv6,
     bool disableServiceAuthCodes, {
-    @required Logger logger,
+    required Logger logger,
   }) async {
     final Uri ddsUri = Uri(
       scheme: 'http',
@@ -45,7 +43,7 @@ class DartDevelopmentService {
         io.InternetAddress.loopbackIPv6 :
         io.InternetAddress.loopbackIPv4
       ).host,
-      port: hostPort ?? 0,
+      port: hostPort,
     );
     logger.printTrace(
       'Launching a Dart Developer Service (DDS) instance at $ddsUri, '
@@ -58,12 +56,12 @@ class DartDevelopmentService {
           enableAuthCodes: !disableServiceAuthCodes,
           ipv6: ipv6,
         );
-      unawaited(_ddsInstance.done.whenComplete(() {
+      unawaited(_ddsInstance?.done.whenComplete(() {
         if (!_completer.isCompleted) {
           _completer.complete();
         }
       }));
-      logger.printTrace('DDS is listening at ${_ddsInstance.uri}.');
+      logger.printTrace('DDS is listening at ${_ddsInstance?.uri}.');
     } on dds.DartDevelopmentServiceException catch (e) {
       logger.printTrace('Warning: Failed to start DDS: ${e.message}');
       if (e.errorCode == dds.DartDevelopmentServiceException.existingDdsInstanceError) {