Noop.

Rename f -> fileAst.
Rename fs -> fset in a comment.
Use ast.Inspect() instead of FindAll() in NewTest4Widget.

Speedup from ~25-35 ms to < 0.01 ms per search.
It prunes unwanted branches, main source of speedup.
Results in a sorted slice, making finding the smallest element trivial.
Works with ast.Node types.
It's a part of std lib.