`KeyValuePipe` currently accepts `null` values as well as `Map`s and a
few others. However, due to the way in which TS overloads work, a type
of `T|null` will not be accepted by `KeyValuePipe`'s signatures, even
though both `T` and `null` individually would be.
To make this work, each signature that accepts some type `T` has been
duplicated with a second one below it that accepts a `T|null` and
includes `null` in its return type.
Fixes#35743
PR Close#36093
Moves the public api .d.ts files from tools/public_api_guard to
goldens/public-api.
Additionally, provides a README in the goldens directory and a script
assist in testing the current state of the repo against the goldens as
well as a command for accepting all changes to the goldens in a single
command.
PR Close#35768
JoostK
Joey Perrott