M, R> typedef
Signature for the callback passed to compute.
message given to it as well as the result have to be
objects that can be sent across isolates (as they may be transitively copied
if needed). The majority of objects can be sent across isolates.
See SendPort.send for more information about exceptions as well as a note of warning about sending closures, which can capture more state than needed.
typedef ComputeCallback<M, R> = FutureOr<R> Function(M message);