properties to require actor isolation.
Member initializer expressions are only used in a constructor with
matching actor isolation. If the isolation prohibits the member
initializer from being evaluated synchronously (or propagating required
isolation through closure bodies), then the default value cannot be used
and the member must be explicitly initialized in the constructor.
Member initializer expressions are also used as default arguments for the
memberwise initializer, and the same rules for default argument isolation
apply.
