Ability to deprecate an export

[lrhn]

We can currently @Deprecate(...) a declaration, which should warn for any code that would change behavior if that declaration was removed.
I would like to be able to deprecate exports, so that code would get warned if they rely on that export for access to a declaration (they'd break if the export was removed, just as for declarations).

My use-case is that I want to move a declaration to another library. To do that, I want the same declaration to be temporarily available from both libraries, one deprecated and the other not.
Since it must be the same declaration (otherwise, their names would clash), I can't annotate the declaration itself without deprecating both sources.

So, instead I'd put the declaration into either one of the two libraries, or a shared third library, and export it deprecated from one library and not from the other.
Any user who imports both should get no warning (they have a non-deprecated path to the declaration).
Any user who imports only through the deprecated export will get a warning.

Eventually, the deprecated export goes away, and all is well.

(This has happened already with BytesBuilder moving from dart:io to dart:typed_data. There is a non-functional deprecation on the export in dart:io. It's happening again to me with another feature, hence this request.)

[srawlins]

Looks like a duplicate of #23067.

[lrhn]

Indeed.