Revert "feat: migrate base/dds.dart to null-safety (#88015)" (#88367)

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

diff --git a/packages/flutter_tools/lib/src/base/dds.dart b/packages/flutter_tools/lib/src/base/dds.dart
index 48b306f..e7c188b 100644
--- a/packages/flutter_tools/lib/src/base/dds.dart
+++ b/packages/flutter_tools/lib/src/base/dds.dart
@@ -2,6 +2,8 @@
 // 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;
@@ -22,10 +24,10 @@
 /// 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>();
@@ -35,7 +37,7 @@
     int hostPort,
     bool ipv6,
     bool disableServiceAuthCodes, {
-    required Logger logger,
+    @required Logger logger,
   }) async {
     final Uri ddsUri = Uri(
       scheme: 'http',
@@ -43,7 +45,7 @@
         io.InternetAddress.loopbackIPv6 :
         io.InternetAddress.loopbackIPv4
       ).host,
-      port: hostPort,
+      port: hostPort ?? 0,
     );
     logger.printTrace(
       'Launching a Dart Developer Service (DDS) instance at $ddsUri, '
@@ -56,12 +58,12 @@
           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) {