Fix perfetto integration test goldens (#7836)

packages/devtools_app/lib/src/screens/performance/panes/timeline_events/perfetto/_perfetto_web.dart

@@ -101,6 +101,13 @@ class _PerfettoViewController extends DisposableController
   /// 'onLoad' stream.
   late final Completer<void> _perfettoIFrameReady;
 
+  /// Whether the perfetto iFrame has been unloaded after loading.
+  ///
+  /// This is stored to prevent race conditions where the iFrame's content
+  /// window has become null. This is set to true when the perfetto iFrame has
+  /// received the first event on the 'unload' stream.
+  bool _perfettoIFrameUnloaded = false;
+
   /// Completes when the Perfetto postMessage handler is ready, which is
   /// signaled by receiving a [_perfettoPong] event in response to sending a
   /// [_perfettoPing] event.
@@ -132,13 +139,25 @@ class _PerfettoViewController extends DisposableController
     _perfettoIFrameReady = Completer<void>();
     _perfettoHandlerReady = Completer<void>();
     _devtoolsThemeHandlerReady = Completer<void>();
+    _perfettoIFrameUnloaded = false;
 
     unawaited(
       perfettoController.perfettoIFrame.onLoad.first.then((_) {
         _perfettoIFrameReady.complete();
       }),
     );
 
+    // TODO(kenz): uncomment once https://github.com/dart-lang/web/pull/246 is
+    // landed and package:web 0.6.0 is published.
+    // unawaited(
+    //   perfettoController.perfettoIFrame.onUnload.first.then((_) {
+    //     if (_perfettoIFrameReady.isCompleted) {
+    //       // Only set to true if this occurs after the iFrame has been loaded.
+    //       _perfettoIFrameUnloaded = true;
+    //     }
+    //   }),
+    // );
+
     window.addEventListener(
       'message',
       _handleMessageListener = _handleMessage.toJS,
@@ -242,6 +261,7 @@ class _PerfettoViewController extends DisposableController
 
   void _postMessage(Object message) async {
     await _perfettoIFrameReady.future;
+    if (_perfettoIFrameUnloaded) return;
     assert(
       perfettoController.perfettoIFrame.contentWindow != null,
       'Something went wrong. The iFrame\'s contentWindow is null after the'
@@ -323,7 +343,9 @@ class _PerfettoViewController extends DisposableController
     window.removeEventListener('message', _handleMessageListener);
     _handleMessageListener = null;
     _pollForPerfettoHandlerReady?.cancel();
+    _pollForPerfettoHandlerReady = null;
     _pollForThemeHandlerReady?.cancel();
+    _pollForThemeHandlerReady = null;
     super.dispose();
   }
 }