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migrate some cupertino files to nullsafety (#66024)

* migrate some cupertino files to nullsafety

* address review comments

* address review comments
A
Alexandre Ardhuin committed
45fa60eb3e58f19bddd8df63e9bbf08fb2ced0b8
Parent: 71c1f6c
Committed by GitHub <noreply@github.com> on 9/23/2020, 4:46:30 AM