Fix implicit return assertions after statements

[szw9999]

Fixes #2.

Summary

• Fix implicit return detection so non-empty annotated functions ending without return still receive a generated return assertion.
• Add a regression fixture covering a function body with a statement and no explicit return.

Verification

• git diff --check

I could not run the full Mocha suite locally because npm registry requests timed out while installing this legacy dependency tree in the available environment.