[flutter_tools] attempt to stabilize hot restart benchmark the old fashioned way (#67971)

A change which sped up hot restart locally caused many of the devicelab measures to regress. I think this is because we do not measure when the isolate is actually "ready", so starting a reload or restart prematurely can cause time spent doing initialization to be registered as part of the reload operation.

A fix for this would be to have the framework include some sort of "initialization complete" event ... but it is not clear what the correct trigger would be. Perhaps after the first frame is successfully registered?

(9a3a0dc1 caused the benchmark regression - possibly since we spend less time syncing files now so we start the restart earlier)

dev/devicelab/lib/tasks/hot_mode_tests.dart

@@ -26,7 +26,7 @@ TaskFunction createHotModeTest({String deviceIdOverride, Map<String, String> env
     final File benchmarkFile = file(path.join(_editedFlutterGalleryDir.path, 'hot_benchmark.json'));
     rm(benchmarkFile);
     final List<String> options = <String>[
-      '--hot', '-d', deviceIdOverride, '--benchmark', '--verbose', '--resident', '--output-dill', path.join('build', 'app.dill')
+      '--hot', '-d', deviceIdOverride, '--benchmark', '--verbose', '--resident', '--output-dill',
     ];
     int hotReloadCount = 0;
     Map<String, dynamic> twoReloadsData;

packages/flutter_tools/lib/src/run_hot.dart

@@ -237,7 +237,6 @@ class HotRunner extends ResidentRunner {
     if (debuggingOptions.fastStart) {
       await restart(
         fullRestart: true,
-        benchmarkMode: !debuggingOptions.startPaused,
         reason: 'restart',
         silent: true,
       );
@@ -246,14 +245,18 @@ class HotRunner extends ResidentRunner {
     appStartedCompleter?.complete();
 
     if (benchmarkMode) {
+      // Wait multiple seconds for the isolate to have fully started.
+      await Future<void>.delayed(const Duration(seconds: 10));
       // We are running in benchmark mode.
       globals.printStatus('Running in benchmark mode.');
       // Measure time to perform a hot restart.
       globals.printStatus('Benchmarking hot restart');
-      await restart(fullRestart: true, benchmarkMode: true);
-      // Wait for notifications to finish. attempt to work around
-      // timing issue caused by sentinel.
-      await Future<void>.delayed(const Duration(seconds: 1));
+      await restart(fullRestart: true);
+      // Wait multiple seconds to stabilize benchmark on slower devicelab hardware.
+      // Hot restart finishes when the new isolate is started, not when the new isolate
+      // is ready. This process can actually take multiple seconds.
+      await Future<void>.delayed(const Duration(seconds: 10));
+
       globals.printStatus('Benchmarking hot reload');
       // Measure time to perform a hot reload.
       await restart(fullRestart: false);
@@ -462,22 +465,10 @@ class HotRunner extends ResidentRunner {
                           deviceAssetsDirectoryUri));
     }
     await Future.wait(futures);
-    if (benchmarkMode) {
-      futures.clear();
-      for (final FlutterDevice device in flutterDevices) {
-        final List<FlutterView> views = await device.vmService.getFlutterViews();
-        for (final FlutterView view in views) {
-          futures.add(device.vmService
-            .flushUIThreadTasks(uiIsolateId: view.uiIsolate.id));
-        }
-      }
-      await Future.wait(futures);
-    }
   }
 
   Future<OperationResult> _restartFromSources({
     String reason,
-    bool benchmarkMode = false,
   }) async {
     final Stopwatch restartTimer = Stopwatch()..start();
     // TODO(aam): Add generator reset logic once we switch to using incremental
@@ -562,32 +553,6 @@ class HotRunner extends ResidentRunner {
     // Send timing analytics.
     globals.flutterUsage.sendTiming('hot', 'restart', restartTimer.elapsed);
 
-    if (benchmarkMode) {
-      final List<Future<void>> isolateNotifications = <Future<void>>[];
-      for (final FlutterDevice device in flutterDevices) {
-        try {
-          await device.vmService.streamListen('Isolate');
-        } on vm_service.RPCError {
-          // Do nothing, we're already subscribed.
-        }
-        isolateNotifications.add(
-          device.vmService.onIsolateEvent.firstWhere((vm_service.Event event) {
-            return event.kind == vm_service.EventKind.kIsolateRunnable;
-          }),
-        );
-      }
-      await Future.wait(isolateNotifications);
-      final List<Future<void>> futures = <Future<void>>[];
-      for (final FlutterDevice device in flutterDevices) {
-        final List<FlutterView> views = await device.vmService.getFlutterViews();
-        for (final FlutterView view in views) {
-          futures.add(device.vmService
-            .flushUIThreadTasks(uiIsolateId: view.uiIsolate.id));
-        }
-      }
-      await Future.wait(futures);
-    }
-
     // Toggle the main dill name after successfully uploading.
     _swap =! _swap;
 
@@ -626,7 +591,6 @@ class HotRunner extends ResidentRunner {
   Future<OperationResult> restart({
     bool fullRestart = false,
     String reason,
-    bool benchmarkMode = false,
     bool silent = false,
     bool pause = false,
   }) async {
@@ -658,7 +622,6 @@ class HotRunner extends ResidentRunner {
         sdkName: sdkName,
         emulator: emulator,
         reason: reason,
-        benchmarkMode: benchmarkMode,
         silent: silent,
       );
       if (!silent) {
@@ -687,7 +650,6 @@ class HotRunner extends ResidentRunner {
     String sdkName,
     bool emulator,
     String reason,
-    bool benchmarkMode,
     bool silent,
   }) async {
     if (!canHotRestart) {
@@ -713,7 +675,6 @@ class HotRunner extends ResidentRunner {
       // handling, at least until we can refactor the underlying code.
       result = await asyncGuard(() => _restartFromSources(
         reason: reason,
-        benchmarkMode: benchmarkMode,
       ));
       if (!result.isOk) {
         restartEvent = 'restart-failed';