isAssignableFromType throws null check error for Function types

The implementation null asserts `DartType.element2`:
https://github.com/dart-lang/source_gen/blob/0d4c54d2a299643ba2a783ef2e9fcf878305a62d/source_gen/lib/src/type_checker.dart#L170-L171

Which is always null for Function types:
https://github.com/dart-lang/sdk/blob/d6ff1193c5eb1d4b98e232f545cce49c63492b5c/pkg/analyzer/lib/src/dart/element/type.dart#L95-L96

Start in source_gen/lib/src/type_checker.dart at the null assertion in isAssignableFromType, then compare the referenced analyzer DartType implementation for Function types. Reproduce the failure with a Function type and confirm the completed change no longer throws a null-check error while preserving assignability behavior.

Written by the indexing model from the issue text.