Use ?. over explicit null check (dart-lang/watcher#85)

diff --git a/pkgs/watcher/lib/src/directory_watcher/linux.dart b/pkgs/watcher/lib/src/directory_watcher/linux.dart
index 0a66a12..7348cb4 100644
--- a/pkgs/watcher/lib/src/directory_watcher/linux.dart
+++ b/pkgs/watcher/lib/src/directory_watcher/linux.dart

@@ -262,7 +262,7 @@
     StreamSubscription subscription;
     subscription = stream.listen(onData, onError: onError, onDone: () {
       _subscriptions.remove(subscription);
-      if (onDone != null) onDone();
+      onDone?.call();
     }, cancelOnError: cancelOnError);
     _subscriptions.add(subscription);
   }

diff --git a/pkgs/watcher/lib/src/directory_watcher/mac_os.dart b/pkgs/watcher/lib/src/directory_watcher/mac_os.dart
index 0aa0566..4408ff1 100644
--- a/pkgs/watcher/lib/src/directory_watcher/mac_os.dart
+++ b/pkgs/watcher/lib/src/directory_watcher/mac_os.dart

@@ -95,8 +95,8 @@
 
   @override
   void close() {
-    if (_watchSubscription != null) _watchSubscription.cancel();
-    if (_initialListSubscription != null) _initialListSubscription.cancel();
+    _watchSubscription?.cancel();
+    _initialListSubscription?.cancel();
     _watchSubscription = null;
     _initialListSubscription = null;
 
@@ -365,7 +365,7 @@
   /// of its state.
   Future _listDir() {
     assert(!isReady);
-    if (_initialListSubscription != null) _initialListSubscription.cancel();
+    _initialListSubscription?.cancel();
 
     _files.clear();
     var completer = Completer();

diff --git a/pkgs/watcher/lib/src/directory_watcher/polling.dart b/pkgs/watcher/lib/src/directory_watcher/polling.dart
index 388f28a..902e8b8 100644
--- a/pkgs/watcher/lib/src/directory_watcher/polling.dart
+++ b/pkgs/watcher/lib/src/directory_watcher/polling.dart

@@ -92,7 +92,7 @@
     _events.close();
 
     // If we're in the middle of listing the directory, stop.
-    if (_listSubscription != null) _listSubscription.cancel();
+    _listSubscription?.cancel();
 
     // Don't process any remaining files.
     _filesToProcess.clear();

diff --git a/pkgs/watcher/lib/src/directory_watcher/windows.dart b/pkgs/watcher/lib/src/directory_watcher/windows.dart
index 9a7de5f..f5093c5 100644
--- a/pkgs/watcher/lib/src/directory_watcher/windows.dart
+++ b/pkgs/watcher/lib/src/directory_watcher/windows.dart

@@ -32,9 +32,7 @@
 
   void addEvent(FileSystemEvent event, void Function() callback) {
     events.add(event);
-    if (timer != null) {
-      timer.cancel();
-    }
+    timer?.cancel();
     timer = Timer(_BATCH_DELAY, callback);
   }
 
@@ -102,9 +100,9 @@
 
   @override
   void close() {
-    if (_watchSubscription != null) _watchSubscription.cancel();
-    if (_parentWatchSubscription != null) _parentWatchSubscription.cancel();
-    if (_initialListSubscription != null) _initialListSubscription.cancel();
+    _watchSubscription?.cancel();
+    _parentWatchSubscription?.cancel();
+    _initialListSubscription?.cancel();
     for (var sub in _listSubscriptions) {
       sub.cancel();
     }
@@ -401,7 +399,7 @@
   /// of its state.
   Future<void> _listDir() {
     assert(!isReady);
-    if (_initialListSubscription != null) _initialListSubscription.cancel();
+    _initialListSubscription?.cancel();
 
     _files.clear();
     var completer = Completer();

diff --git a/pkgs/watcher/lib/src/file_watcher/native.dart b/pkgs/watcher/lib/src/file_watcher/native.dart
index 7f466af..0b42cb9 100644
--- a/pkgs/watcher/lib/src/file_watcher/native.dart
+++ b/pkgs/watcher/lib/src/file_watcher/native.dart

@@ -83,7 +83,7 @@
 
   @override
   void close() {
-    if (_subscription != null) _subscription.cancel();
+    _subscription?.cancel();
     _subscription = null;
     _eventsController.close();
   }