Pre SIP: Replace non-sensical @unchecked annotations

[bishabosha]

@chrisandrews-ms

This gives a warning about narrowing, as expected, but it suggests annotating with @unchecked rather than using .runtimeChecked. Should that message be changed? Perhaps we will change it only when runtimeChecked becomes non-experimental?

when I implemented it I asked about this and the recommendation was to leave everything untouched, (i.e only implement runtimeChecked as a non-default alternative) until comes such a point that we plan to require it over @unchecked