CastMap does an invalid cast on 'remove'

[stuartmorgan]

STR:

  final foo = json.decode('{}').cast<String, Object>();
  foo.remove('akey');

sdk/sdk/lib/internal/cast.dart

Line 280 in d44457f

V remove(Object? key) => _source.remove(key) as V;

is incorrect, since remove returns a nullable value even if the value type is non-null.

[stuartmorgan]

This is causing issues for the very popular shared_preferences plugin on Linux and Windows, so ideally we would cherry-pick a fix for this to stable. (If that's not feasible, we can work around it in the plugin, but I don't think that plugin is going to be the only thing to hit this.)

[devoncarew]

cc @lrhn

[lrhn]

Fixed by https://dart-review.googlesource.com/c/sdk/+/194244

[stuartmorgan]

@lrhn Is it possible to cherry-pick this since it's a very straightforward fix?

[lrhn]

Shouldn't be a problem. Not sure where we are in release cycles right now, but if it's cherry-pick season, it would be nice to get it in.