Sends a data event.
Listeners receive this event in a later microtask.
Note that a synchronous controller (created by passing true to the sync
parameter of the StreamController constructor) delivers events
immediately. Since this behavior violates the contract mentioned here,
synchronous controllers should only be used as described in the
documentation to ensure that the delivered events always appear as if
they were delivered in a separate microtask.
Source
void add(T event);