Bizarre text on `style as InternalStyle?` in package:path/src/context.dart

[srawlins]

Steps to reproduce:

1. Check out path package at 1.6.2 (git checkout 1.6.2).
2. Migrate to NNBD (e.g. dart ~/code/dart-sdk/sdk/pkg/dartfix/bin/dartfix.dart --fix=non-nullable --overwrite --outputDir nnbd-i13n .).
3. Open the migration output for lib/src/context.dart. Hover over the inserted ? in style as InternalStyle?, current);

The only reason text is:

This function returns a nullable value

But this has nothing to do with it. The issue is that there is a return statement up the AST, so we need more refined reasoning here.