migrate from slow async io calls (#39515)

* migrate from slow async io calls

dev/devicelab/lib/tasks/perf_tests.dart

@@ -122,7 +122,7 @@ TaskFunction createBasicMaterialCompileTest() {
       await flutter('create', options: <String>['--template=app', sampleAppName]);
     });
 
-    if (!(await sampleDir.exists()))
+    if (!sampleDir.existsSync())
       throw 'Failed to create default Flutter app in ${sampleDir.path}';
 
     return CompileTest(sampleDir.path).run();

packages/flutter_test/test/widget_tester_test.dart

@@ -467,6 +467,7 @@ void main() {
       final StringBuffer buf = StringBuffer('1');
       await tester.runAsync(() async {
         buf.write('2');
+        //ignore: avoid_slow_async_io
         await Directory.current.stat();
         buf.write('3');
       });