apply lint prefer_void_to_null in packages/fuchsia_remote_debug_protocol (#22690)

diff --git a/packages/fuchsia_remote_debug_protocol/examples/drive_todo_list_scroll.dart b/packages/fuchsia_remote_debug_protocol/examples/drive_todo_list_scroll.dart
index 0445104..bfe954b 100644
--- a/packages/fuchsia_remote_debug_protocol/examples/drive_todo_list_scroll.dart
+++ b/packages/fuchsia_remote_debug_protocol/examples/drive_todo_list_scroll.dart

@@ -22,7 +22,7 @@
 ///
 /// $ dart examples/driver_todo_list_scroll.dart \
 ///     fe80::8eae:4cff:fef4:9247 eno1
-Future<Null> main(List<String> args) async {
+Future<void> main(List<String> args) async {
   // Log only at info level within the library. If issues arise, this can be
   // changed to [LoggingLevel.all] or [LoggingLevel.fine] to see more
   // information.
@@ -56,7 +56,7 @@
     // Scrolls down 300px.
     await driver.scroll(find.byType('Scaffold'), 0.0, -300.0,
         const Duration(milliseconds: 300));
-    await Future<Null>.delayed(const Duration(milliseconds: 500));
+    await Future<void>.delayed(const Duration(milliseconds: 500));
     // Scrolls up 300px.
     await driver.scroll(find.byType('Scaffold'), 300.0, 300.0,
         const Duration(milliseconds: 300));

diff --git a/packages/fuchsia_remote_debug_protocol/examples/list_vms_and_flutter_views.dart b/packages/fuchsia_remote_debug_protocol/examples/list_vms_and_flutter_views.dart
index 6e3ac67..3a4ff2d 100644
--- a/packages/fuchsia_remote_debug_protocol/examples/list_vms_and_flutter_views.dart
+++ b/packages/fuchsia_remote_debug_protocol/examples/list_vms_and_flutter_views.dart

@@ -18,7 +18,7 @@
 ///
 /// $ dart examples/list_vms_and_flutter_views.dart \
 ///     fe80::8eae:4cff:fef4:9247 eno1
-Future<Null> main(List<String> args) async {
+Future<void> main(List<String> args) async {
   // Log only at info level within the library. If issues arise, this can be
   // changed to [LoggingLevel.all] or [LoggingLevel.fine] to see more
   // information.

diff --git a/packages/fuchsia_remote_debug_protocol/lib/src/dart/dart_vm.dart b/packages/fuchsia_remote_debug_protocol/lib/src/dart/dart_vm.dart
index 3f81acf..178ad9e 100644
--- a/packages/fuchsia_remote_debug_protocol/lib/src/dart/dart_vm.dart
+++ b/packages/fuchsia_remote_debug_protocol/lib/src/dart/dart_vm.dart

@@ -55,7 +55,7 @@
       await socket?.close();
       if (timer.elapsed < _kConnectTimeout) {
         _log.info('Attempting to reconnect');
-        await Future<Null>.delayed(_kReconnectAttemptInterval);
+        await Future<void>.delayed(_kReconnectAttemptInterval);
         return attemptConnection(uri);
       } else {
         _log.warning('Connection to Fuchsia\'s Dart VM timed out at '
@@ -181,7 +181,7 @@
   /// Disconnects from the Dart VM Service.
   ///
   /// After this function completes this object is no longer usable.
-  Future<Null> stop() async {
+  Future<void> stop() async {
     await _peer?.close();
   }
 }

diff --git a/packages/fuchsia_remote_debug_protocol/lib/src/fuchsia_remote_connection.dart b/packages/fuchsia_remote_debug_protocol/lib/src/fuchsia_remote_connection.dart
index 1fec7ca..fba3566 100644
--- a/packages/fuchsia_remote_debug_protocol/lib/src/fuchsia_remote_connection.dart
+++ b/packages/fuchsia_remote_debug_protocol/lib/src/fuchsia_remote_connection.dart

@@ -135,10 +135,10 @@
     await connection._forwardLocalPortsToDeviceServicePorts();
 
     Stream<DartVmEvent> dartVmStream() {
-      Future<Null> listen() async {
+      Future<void> listen() async {
         while (connection._pollDartVms) {
           await connection._pollVms();
-          await Future<Null>.delayed(_kVmPollInterval);
+          await Future<void>.delayed(_kVmPollInterval);
         }
         connection._dartVmEventController.close();
       }
@@ -218,7 +218,7 @@
   /// Any objects that this class returns (including any child objects from
   /// those objects) will subsequently have its connection closed as well, so
   /// behavior for them will be undefined.
-  Future<Null> stop() async {
+  Future<void> stop() async {
     for (PortForwarder pf in _forwardedVmServicePorts) {
       // Closes VM service first to ensure that the connection is closed cleanly
       // on the target before shutting down the forwarding itself.
@@ -369,7 +369,7 @@
     final List<E> result = <E>[];
 
     // Helper function loop.
-    Future<Null> shutDownPortForwarder(PortForwarder pf) async {
+    Future<void> shutDownPortForwarder(PortForwarder pf) async {
       await pf.stop();
       _stalePorts.add(pf.remotePort);
       if (queueEvents) {
@@ -432,7 +432,7 @@
   ///
   /// If there are new instances of the Dart VM, then connections will be
   /// attempted (after clearing out stale connections).
-  Future<Null> _pollVms() async {
+  Future<void> _pollVms() async {
     await _checkPorts();
     final List<int> servicePorts = await getDeviceServicePorts();
     for (int servicePort in servicePorts) {
@@ -456,7 +456,7 @@
   /// Runs a dummy heartbeat command on all Dart VM instances.
   ///
   /// Removes any failing ports from the cache.
-  Future<Null> _checkPorts([bool queueEvents = true]) async {
+  Future<void> _checkPorts([bool queueEvents = true]) async {
     // Filters out stale ports after connecting. Ignores results.
     await _invokeForAllVms<Map<String, dynamic>>(
       (DartVm vmService) async {
@@ -473,7 +473,7 @@
   ///
   /// When this function is run, all existing forwarded ports and connections
   /// are reset by way of [stop].
-  Future<Null> _forwardLocalPortsToDeviceServicePorts() async {
+  Future<void> _forwardLocalPortsToDeviceServicePorts() async {
     await stop();
     final List<int> servicePorts = await getDeviceServicePorts();
     final List<PortForwarder> forwardedVmServicePorts =
@@ -543,7 +543,7 @@
   int get remotePort;
 
   /// Shuts down and cleans up port forwarding.
-  Future<Null> stop();
+  Future<void> stop();
 }
 
 /// Instances of this class represent a running SSH tunnel.
@@ -628,7 +628,7 @@
   /// Kills the SSH forwarding command, then to ensure no ports are forwarded,
   /// runs the SSH 'cancel' command to shut down port forwarding completely.
   @override
-  Future<Null> stop() async {
+  Future<void> stop() async {
     // Cancel the forwarding request. See [start] for commentary about why this
     // uses the IPv4 loopback.
     final String formattedForwardingUrl =

diff --git a/packages/fuchsia_remote_debug_protocol/test/fuchsia_remote_connection_test.dart b/packages/fuchsia_remote_debug_protocol/test/fuchsia_remote_connection_test.dart
index 7d266d3..b282f17 100644
--- a/packages/fuchsia_remote_debug_protocol/test/fuchsia_remote_connection_test.dart
+++ b/packages/fuchsia_remote_debug_protocol/test/fuchsia_remote_connection_test.dart

@@ -147,7 +147,7 @@
     });
 
     test('env variable test without remote addr', () async {
-      Future<Null> failingFunction() async {
+      Future<void> failingFunction() async {
         await FuchsiaRemoteConnection.connect();
       }
 

diff --git a/packages/fuchsia_remote_debug_protocol/test/src/dart/dart_vm_test.dart b/packages/fuchsia_remote_debug_protocol/test/src/dart/dart_vm_test.dart
index 9df970f..31d1d32 100644
--- a/packages/fuchsia_remote_debug_protocol/test/src/dart/dart_vm_test.dart
+++ b/packages/fuchsia_remote_debug_protocol/test/src/dart/dart_vm_test.dart

@@ -199,7 +199,7 @@
       final DartVm vm =
           await DartVm.connect(Uri.parse('http://whatever.com/ws'));
       expect(vm, isNot(null));
-      Future<Null> failingFunction() async {
+      Future<void> failingFunction() async {
         await vm.getAllFlutterViews();
       }
 
@@ -279,7 +279,7 @@
       final DartVm vm =
           await DartVm.connect(Uri.parse('http://whatever.com/ws'));
       expect(vm, isNot(null));
-      Future<Null> failingFunction() async {
+      Future<void> failingFunction() async {
         await vm.getAllFlutterViews();
       }
 
@@ -312,7 +312,7 @@
       final DartVm vm =
           await DartVm.connect(Uri.parse('http://whatever.com/ws'));
       expect(vm, isNot(null));
-      Future<Null> failingFunction() async {
+      Future<void> failingFunction() async {
         await vm.invokeRpc('somesillyfunction', timeout: timeoutTime);
       }
 

diff --git a/packages/fuchsia_remote_debug_protocol/test/src/runners/ssh_command_runner_test.dart b/packages/fuchsia_remote_debug_protocol/test/src/runners/ssh_command_runner_test.dart
index 29554d7..46c4fc6 100644
--- a/packages/fuchsia_remote_debug_protocol/test/src/runners/ssh_command_runner_test.dart
+++ b/packages/fuchsia_remote_debug_protocol/test/src/runners/ssh_command_runner_test.dart

@@ -96,7 +96,7 @@
           address: addr);
       when<String>(mockProcessResult.stdout).thenReturn('whatever');
       when(mockProcessResult.exitCode).thenReturn(1);
-      Future<Null> failingFunction() async {
+      Future<void> failingFunction() async {
         await runner.run('oihaw');
       }