fix upcoming lint avoid_returning_null_for_void (#23190)

diff --git a/dev/bots/run_command.dart b/dev/bots/run_command.dart
index 6b8f689..c996aae 100644
--- a/dev/bots/run_command.dart
+++ b/dev/bots/run_command.dart

@@ -44,7 +44,7 @@
   final String relativeWorkingDir = path.relative(workingDirectory);
   if (skip) {
     printProgress('SKIPPING', relativeWorkingDir, commandDescription);
-    return null;
+    return;
   }
   printProgress('RUNNING', relativeWorkingDir, commandDescription);
 

diff --git a/dev/devicelab/bin/run.dart b/dev/devicelab/bin/run.dart
index 102a6df..9ec6b07 100644
--- a/dev/devicelab/bin/run.dart
+++ b/dev/devicelab/bin/run.dart

@@ -28,7 +28,7 @@
     stderr.writeln('Usage:\n');
     stderr.writeln(_argParser.usage);
     exitCode = 1;
-    return null;
+    return;
   }
 
   if (!args.wasParsed('task')) {
@@ -50,7 +50,7 @@
   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'];

diff --git a/dev/devicelab/lib/framework/adb.dart b/dev/devicelab/lib/framework/adb.dart
index 6afac24..04f2bd9 100644
--- a/dev/devicelab/lib/framework/adb.dart
+++ b/dev/devicelab/lib/framework/adb.dart

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