[private-named-parameters] Migrate SDK to use private named parameters

[munificent]

If there are places in the core libraries and Dart-team maintained packages that could be migrated to use private named parameters, we should. It will be good dogfooding, and it ensures that users who read our code will see the idioms we want them to use.

[lrhn]

Definitely requires DartDoc to be private-parameter-name aware first.

[munificent]

Yup, that's dart-lang/dartdoc#4114.