[devicelab] dont use set modified for hot reload bench (#70965)

dev/devicelab/lib/tasks/hot_mode_tests.dart

@@ -19,6 +19,11 @@ const String kSourceLine = 'fontSize: (orientation == Orientation.portrait) ? 32
 const String kReplacementLine = 'fontSize: (orientation == Orientation.portrait) ? 34.0 : 24.0';
 
 TaskFunction createHotModeTest({String deviceIdOverride, Map<String, String> environment}) {
+  // This file is modified during the test and needs to be restored at the end.
+  final File flutterFrameworkSource = file(path.join(
+    flutterDirectory.path, 'packages/flutter/lib/src/widgets/framework.dart',
+  ));
+  final String oldContents = flutterFrameworkSource.readAsStringSync();
   return () async {
     if (deviceIdOverride == null) {
       final Device device = await devices.workingDevice;
@@ -42,6 +47,7 @@ TaskFunction createHotModeTest({String deviceIdOverride, Map<String, String> env
       mkdirs(_editedFlutterGalleryDir);
       recursiveCopy(flutterGalleryDir, _editedFlutterGalleryDir);
 
+      try {
         await inDirectory<void>(_editedFlutterGalleryDir, () async {
           smallReloadData = await captureReloadData(options, environment, benchmarkFile, (String line, Process process) {
             if (!line.contains('Reloaded ')) {
@@ -88,10 +94,9 @@ TaskFunction createHotModeTest({String deviceIdOverride, Map<String, String> env
             }
             if (hotReloadCount == 2) {
               // Trigger a framework invalidation (370 libraries) without modifying the source
-            final File flutterFrameworkSource = file(path.join(
-              flutterDirectory.path, 'packages/flutter/lib/src/widgets/framework.dart',
-            ));
-            flutterFrameworkSource.setLastModifiedSync(DateTime.now());
+              flutterFrameworkSource.writeAsStringSync(
+                flutterFrameworkSource.readAsStringSync() + '\n'
+              );
               process.stdin.writeln('r');
               hotReloadCount += 1;
             } else {
@@ -137,6 +142,9 @@ TaskFunction createHotModeTest({String deviceIdOverride, Map<String, String> env
                 json.decode(benchmarkFile.readAsStringSync()) as Map<String, dynamic>;
           }
         });
+      } finally {
+        flutterFrameworkSource.writeAsStringSync(oldContents);
+      }
     });
 
     return TaskResult.success(