beforeUnloadEvent constant
EventStreamProvider<BeforeUnloadEvent> beforeUnloadEvent = const _BeforeUnloadEventStreamProvider<BeforeUnloadEvent>('beforeunload')Static factory designed to expose beforeunload events to event handlers that are not necessarily instances of Window.
See EventStreamProvider for usage information.
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https://api.dartlang.org/stable/1.24.3/dart-html/Window/beforeUnloadEvent-constant.html