Свойство onStart
Stream<SpeechSynthesisEvent> onStartПоток start событий, обрабатываемых этим SpeechSynthesisUtterance.
Исходный код
@DomName('SpeechSynthesisUtterance.onstart')
@DocsEditable()
Stream<SpeechSynthesisEvent> get onStart => startEvent.forTarget(this);
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Licensed under the Creative Commons Attribution-ShareAlike License v4.0.
https://api.dartlang.org/stable/1.24.3/dart-html/SpeechSynthesisUtterance/onStart.html