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Prefer void to null (#22977)

* Future<void> main

* Future<void>.delayed

* prefer_void_to_Null

* address review comments
A
Alexandre Ardhuin committed
0fb84e96c79df2fe7efeaeb06fbd1c2b53f4a84b
Parent: 5efe095
Committed by GitHub <noreply@github.com> on 10/16/2018, 8:03:06 PM