In error reporting on assignability where the expression has type
`Null` we use a special messages that doesn't state that
"'Null' is nullable". Cases where `Null` is part of the type or where
the expression of type `Null` is derived from a collection are not
special cased.
Closes#43998Closes#44093
Change-Id: Ic0c79e6362f2365eb3fe8222ccd2ef8fac4f188d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/170433
Commit-Queue: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Dmitry Stefantsov <dmitryas@google.com>
