Fixes for review comments.

pkgs/watcher/lib/src/directory_watcher/mac_os.dart

diff --git a/pkgs/watcher/lib/src/directory_watcher/mac_os.dart b/pkgs/watcher/lib/src/directory_watcher/mac_os.dart
index e44b7f5..7dc6e7c 100644
--- a/pkgs/watcher/lib/src/directory_watcher/mac_os.dart
+++ b/pkgs/watcher/lib/src/directory_watcher/mac_os.dart
@@ -144,18 +144,20 @@
 
           if (_files.containsDir(path)) continue;
 
-          late StreamSubscription<FileSystemEntity> subscription;
-          subscription = Directory(path).list(recursive: true).listen((entity) {
+          var stream = Directory(path).list(recursive: true);
+          var subscription = stream.listen((entity) {
             if (entity is Directory) return;
             if (_files.contains(path)) return;
 
             _emitEvent(ChangeType.ADD, entity.path);
             _files.add(entity.path);
-          }, onError: (Object e, StackTrace stackTrace) {
-            _emitError(e, stackTrace);
-          }, onDone: () {
-            _listSubscriptions.remove(subscription);
           }, cancelOnError: true);
+          subscription.onDone(() {
+            _listSubscriptions.remove(subscription);
+          });
+          subscription.onError((Object e, StackTrace stackTrace) {
+            _emitError(e, stackTrace);
+          });
           _listSubscriptions.add(subscription);
         } else if (event is FileSystemModifyEvent) {
           assert(!event.isDirectory);

pkgs/watcher/lib/src/directory_watcher/polling.dart

diff --git a/pkgs/watcher/lib/src/directory_watcher/polling.dart b/pkgs/watcher/lib/src/directory_watcher/polling.dart
index 968c9c6..6baa49d 100644
--- a/pkgs/watcher/lib/src/directory_watcher/polling.dart
+++ b/pkgs/watcher/lib/src/directory_watcher/polling.dart
@@ -70,7 +70,7 @@
   /// queue exists to let each of those proceed at their own rate. The lister
   /// will enqueue files as quickly as it can. Meanwhile, files are dequeued
   /// and processed sequentially.
-  late AsyncQueue<String?> _filesToProcess;
+  late final AsyncQueue<String?> _filesToProcess;
 
   /// The set of files that have been seen in the current directory listing.
   ///

pkgs/watcher/lib/src/directory_watcher/windows.dart

diff --git a/pkgs/watcher/lib/src/directory_watcher/windows.dart b/pkgs/watcher/lib/src/directory_watcher/windows.dart
index 7640373..a8200ee 100644
--- a/pkgs/watcher/lib/src/directory_watcher/windows.dart
+++ b/pkgs/watcher/lib/src/directory_watcher/windows.dart
@@ -184,19 +184,20 @@
           if (_files.containsDir(path)) continue;
 
           var stream = Directory(path).list(recursive: true);
-          late StreamSubscription<FileSystemEntity> subscription;
-          subscription = stream.listen((entity) {
+          var subscription = stream.listen((entity) {
             if (entity is Directory) return;
             if (_files.contains(path)) return;
 
             _emitEvent(ChangeType.ADD, entity.path);
             _files.add(entity.path);
-          }, onDone: () {
+          }, cancelOnError: true);
+          subscription.onDone(() {
             _listSubscriptions.remove(subscription);
-          }, onError: (Object e, StackTrace stackTrace) {
+          });
+          subscription.onError((Object e, StackTrace stackTrace) {
             _listSubscriptions.remove(subscription);
             _emitError(e, stackTrace);
-          }, cancelOnError: true);
+          });
           _listSubscriptions.add(subscription);
         } else if (event is FileSystemModifyEvent) {
           if (!event.isDirectory) {

pkgs/watcher/lib/src/file_watcher/polling.dart

diff --git a/pkgs/watcher/lib/src/file_watcher/polling.dart b/pkgs/watcher/lib/src/file_watcher/polling.dart
index b208a4b..9b9ac5b 100644
--- a/pkgs/watcher/lib/src/file_watcher/polling.dart
+++ b/pkgs/watcher/lib/src/file_watcher/polling.dart
@@ -37,7 +37,7 @@
   final _readyCompleter = Completer();
 
   /// The timer that controls polling.
-  late Timer _timer;
+  late final Timer _timer;
 
   /// The previous modification time of the file.
   ///