This change introduces the class `NullTypeImpl` to represent the
special type `Null`, which was previously represented as an ordinary
`InterfaceType`.
This class mirrors the CFE's `NullType` class. Adding it will
facilitate further code sharing between the analyzer and the CFE,
because it will allow code that's shared between the analyzer and CFE
to use `is` tests to recognize the type `Null`.
The new `NullTypeImpl` class always has a nullability suffix of
`NullabilitySuffix.none`, so it is impossible for the type `Null?` to
occur. This is a benign behavioral change; previously there were some
circumstances in which the type `Null?` might occur in the analyzer,
but the type `Null?` behaved the same as `Null` so there was no
user-visible effect. This behavioral change brings the analyzer's
treatment of `Null` into alignment with that of the CFE.
This is the first step in a planned patch series. In follow-up CLs, I
intend to:
- Introduce a public-facing `NullType` for use by clients.
- Change `NullTypeImpl` so that it no longer extends `InterfaceTypeImpl`.
- Add a base class that is common to the analyzer and CFE `NullType`
representations.
Change-Id: I9cea84d8149347bffdee006d544af28c66eba429
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/396320
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Paul Berry
ac6d86c737