A "begin" Flow event.
When passed to a Timeline method, generates a "begin" Flow event. If id
is not provided, an id that conflicts with no other Dart-generated flow id's will be generated.
static Flow begin({int id}) { return new Flow._(_begin, id ?? _getNextAsyncId()); }
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https://api.dartlang.org/stable/2.0.0/dart-developer/Flow/begin.html