fix upcoming lint avoid_returning_null_for_void (#23190)

dev/bots/run_command.dart

@@ -44,7 +44,7 @@ Future<void> runCommand(String executable, List<String> arguments, {
   final String relativeWorkingDir = path.relative(workingDirectory);
   if (skip) {
     printProgress('SKIPPING', relativeWorkingDir, commandDescription);
-    return null;
+    return;
   }
   printProgress('RUNNING', relativeWorkingDir, commandDescription);
 

dev/devicelab/bin/run.dart

@@ -28,7 +28,7 @@ Future<void> main(List<String> rawArgs) async {
     stderr.writeln('Usage:\n');
     stderr.writeln(_argParser.usage);
     exitCode = 1;
-    return null;
+    return;
   }
 
   if (!args.wasParsed('task')) {
@@ -50,7 +50,7 @@ Future<void> main(List<String> rawArgs) async {
   if (_taskNames.isEmpty) {
     stderr.writeln('Failed to find tasks to run based on supplied options.');
     exitCode = 1;
-    return null;
+    return;
   }
 
   final bool silent = args['silent'];

dev/devicelab/lib/framework/adb.dart

@@ -407,7 +407,6 @@ class IosDeviceDiscovery implements DeviceDiscovery {
   @override
   Future<void> performPreflightTasks() async {
     // Currently we do not have preflight tasks for iOS.
-    return null;
   }
 }
 

examples/flutter_gallery/lib/gallery/updater.dart

@@ -34,7 +34,7 @@ class UpdaterState extends State<Updater> {
     // Only prompt once a day
     if (_lastUpdateCheck != null &&
         DateTime.now().difference(_lastUpdateCheck) < const Duration(days: 1)) {
-      return null; // We already checked for updates recently
+      return; // We already checked for updates recently
     }
     _lastUpdateCheck = DateTime.now();
 

packages/flutter/test/rendering/sliver_cache_test.dart

@@ -931,7 +931,7 @@ class TestRenderSliverBoxChildManager extends RenderSliverBoxChildManager {
   @override
   void createChild(int index, { @required RenderBox after }) {
     if (index < 0 || index >= children.length)
-      return null;
+      return;
     try {
       _currentlyUpdatingChildIndex = index;
       _renderObject.insert(children[index], after: after);

packages/flutter/test/rendering/slivers_block_test.dart

@@ -27,7 +27,7 @@ class TestRenderSliverBoxChildManager extends RenderSliverBoxChildManager {
   @override
   void createChild(int index, { @required RenderBox after }) {
     if (index < 0 || index >= children.length)
-      return null;
+      return;
     try {
       _currentlyUpdatingChildIndex = index;
       _renderObject.insert(children[index], after: after);

packages/flutter_test/lib/src/binding.dart

@@ -568,7 +568,6 @@ abstract class TestWidgetsFlutterBinding extends BindingBase
 
     assert(inTest);
     asyncBarrier(); // When using AutomatedTestWidgetsFlutterBinding, this flushes the microtasks.
-    return null;
   }
 
   void _verifyInvariants() {

packages/flutter_tools/bin/fuchsia_asset_builder.dart

@@ -40,7 +40,6 @@ Future<void> writeFile(libfs.File outputFile, DevFSContent content) async {
   outputFile.createSync(recursive: true);
   final List<int> data = await content.contentsAsBytes();
   outputFile.writeAsBytesSync(data);
-  return null;
 }
 
 Future<void> run(List<String> args) async {

packages/flutter_tools/lib/src/cache.dart

@@ -72,7 +72,7 @@ class Cache {
   /// calling [Cache.releaseLockEarly] once they are no longer touching the cache.
   static Future<void> lock() async {
     if (!_lockEnabled)
-      return null;
+      return;
     assert(_lock == null);
     _lock = await fs.file(fs.path.join(flutterRoot, 'bin', 'cache', 'lockfile')).open(mode: FileMode.write);
     bool locked = false;
@@ -201,7 +201,7 @@ class Cache {
 
   Future<void> updateAll() async {
     if (!_lockEnabled)
-      return null;
+      return;
     try {
       for (CachedArtifact artifact in _artifacts) {
         if (!artifact.isUpToDate())

packages/flutter_tools/lib/src/crash_reporting.dart

@@ -87,7 +87,7 @@ class CrashReportSender {
   }) async {
     try {
       if (_usage.suppressAnalytics)
-        return null;
+        return;
 
       printStatus('Sending crash report to Google.');
 

packages/flutter_tools/lib/src/ios/devices.dart

@@ -579,7 +579,7 @@ class _IOSDevicePortForwarder extends DevicePortForwarder {
   Future<void> unforward(ForwardedPort forwardedPort) async {
     if (!_forwardedPorts.remove(forwardedPort)) {
       // Not in list. Nothing to remove.
-      return null;
+      return;
     }
 
     printTrace('Unforwarding port $forwardedPort');
@@ -591,7 +591,5 @@ class _IOSDevicePortForwarder extends DevicePortForwarder {
     } else {
       printError('Forwarded port did not have a valid process');
     }
-
-    return null;
   }
 }

packages/flutter_tools/lib/src/run_cold.dart

@@ -109,7 +109,7 @@ class ColdRunner extends ResidentRunner {
   }
 
   @override
-  Future<void> handleTerminalCommand(String code) async => null;
+  Future<void> handleTerminalCommand(String code) async { }
 
   @override
   Future<void> cleanupAfterSignal() async {

packages/flutter_tools/test/src/mocks.dart

@@ -278,10 +278,10 @@ class MemoryIOSink implements IOSink {
   Future<void> get done => close();
 
   @override
-  Future<void> close() async => null;
+  Future<void> close() async { }
 
   @override
-  Future<void> flush() async => null;
+  Future<void> flush() async { }
 }
 
 /// A Stdio that collects stdout and supports simulated stdin.