Engine startup event timed after VM initializes (flutter#35713)

* Engine startup event timed after VM initializes

* Formatting

* Update FlutterEngineMainEnter event comment

* Remove engine_start_time

* Remove unused variable

Author: yaakovschectman

common/settings.h

@@ -301,11 +301,6 @@ struct Settings {
   // Max bytes threshold of resource cache, or 0 for unlimited.
   size_t resource_cache_max_bytes_threshold = 0;
 
-  /// A timestamp representing when the engine started. The value is based
-  /// on the clock used by the Dart timeline APIs. This timestamp is used
-  /// to log a timeline event that tracks the latency of engine startup.
-  std::chrono::microseconds engine_start_timestamp = {};
-
   /// The minimum number of samples to require in multipsampled anti-aliasing.
   ///
   /// Setting this value to 0 or 1 disables MSAA.

runtime/dart_vm.cc

@@ -467,19 +467,19 @@ DartVM::DartVM(std::shared_ptr<const DartVMData> vm_data,
     // Send the earliest available timestamp in the application lifecycle to
     // timeline. The difference between this timestamp and the time we render
     // the very first frame gives us a good idea about Flutter's startup time.
-    // Use a duration event so about:tracing will consider this event when
-    // deciding the earliest event to use as time 0.
-    if (settings_.engine_start_timestamp.count()) {
-      Dart_TimelineEvent(
-          "FlutterEngineMainEnter",                  // label
-          settings_.engine_start_timestamp.count(),  // timestamp0
-          Dart_TimelineGetMicros(),                  // timestamp1_or_async_id
-          Dart_Timeline_Event_Duration,              // event type
-          0,                                         // argument_count
-          nullptr,                                   // argument_names
-          nullptr                                    // argument_values
-      );
-    }
+    // Use an instant event because the call to Dart_TimelineGetMicros
+    // may behave differently before and after the Dart VM is initialized.
+    // As this call is immediately after initialization of the Dart VM,
+    // we are interested in only one timestamp.
+    int64_t micros = Dart_TimelineGetMicros();
+    Dart_TimelineEvent("FlutterEngineMainEnter",     // label
+                       micros,                       // timestamp0
+                       micros,                       // timestamp1_or_async_id
+                       Dart_Timeline_Event_Instant,  // event type
+                       0,                            // argument_count
+                       nullptr,                      // argument_names
+                       nullptr                       // argument_values
+    );
   }
 
   Dart_SetFileModifiedCallback(&DartFileModifiedCallback);

shell/common/shell.cc

@@ -84,11 +84,6 @@ void PerformInitializationTasks(Settings& settings) {
 
   static std::once_flag gShellSettingsInitialization = {};
   std::call_once(gShellSettingsInitialization, [&settings] {
-    if (settings.engine_start_timestamp.count() == 0) {
-      settings.engine_start_timestamp =
-          std::chrono::microseconds(Dart_TimelineGetMicros());
-    }
-
     tonic::SetLogHandler(
         [](const char* message) { FML_LOG(ERROR) << message; });
 

shell/platform/android/flutter_main.cc

@@ -111,10 +111,6 @@ void FlutterMain::Init(JNIEnv* env,
   settings.enable_timeline_event_handler = settings.trace_systrace;
 #endif  // FLUTTER_RELEASE
 
-  int64_t init_time_micros = initTimeMillis * 1000;
-  settings.engine_start_timestamp =
-      std::chrono::microseconds(Dart_TimelineGetMicros() - init_time_micros);
-
   // Restore the callback cache.
   // TODO(chinmaygarde): Route all cache file access through FML and remove this
   // setter.