add a dart fix declarative refactoring for the whereNotNull() => nonNulls change

[devoncarew]

See dart-archive/collection#332.

I believe this would be a 'replacedBy' dart fix change.

[oprypin]

In the past there was a limitation preventing this: can't turn a method call into a getter call. Don't know what the latest status is.

[devoncarew]

Yeah, it doesn't appear to be working though it is had to know exactly what syntax to use in the declarative file. I'm guessing that dart fix hasn't seen this exact refactoring before (replacing a deprecated extension method with an extension getter).