To discourage developers from mutating the arrays returned
from the following methods, their return types have been marked
as readonly.
* `getLocaleDayPeriods()`
* `getLocaleDayNames()`
* `getLocaleMonthNames()`
* `getLocaleEraNames()`
Fixes#27003
BREAKING CHANGE:
The locale data API has been marked as returning readonly arrays, rather
than mutable arrays, since these arrays are shared across calls to the
API. If you were mutating them (e.g. calling `sort()`, `push()`, `splice()`, etc)
then your code will not longer compile. If you need to mutate the array, you
should now take a copy (e.g. by calling `slice()`) and mutate the copy.
PR Close#30397
6acea54f62