Can you elaborate in the docs on what this class adds? Specifically, looking at it in isolation, it doesn't add any properties or methods over a HitTestResult, so it makes me wonder what it's used for.

It is correct that SectorHitTestResult currently does not provide any additional functionality over HitTestResult because RenderSector is an incomplete example implementation of an alternative to RenderBox. Once we have something like RenderSectorTransform (which, changes the origin of its child to be different from the parent) we'd add methods here to implement the origin changing for hit testing. I've added added a TODO for this to the class.

Worth redefining HitTestEntry to be HitTestEntry<T extends HitTestTarget> and making this class be SectorHitTestEntry extends HitTestEntry<RenderSector>?

That probably would have been a good idea when HitTestEntry was first written. This is now just copying what the existing BoxHitTestEntry and SliverHitTestEntry is doing. Changing this now would be another breaking change. This one I find really hard to justify since it doesn't provide any user value. I'll leave it as is for now.

assert(radius > 0), assert(theta > 0)?

I believe negative values for these are fine.

Can you elaborate on what the theta component is or provide breadcrumbs to more info about hit testing that explains the concepts of radius and theta?

Radius and theta are concepts of a polar coordinate system, which is implemented by RenderSector as an alternative to RenderBox, which implements a cartesian coordinate system. It is just an example (see the directory) and as such poorly documented. (These are in fact the first doc comments in the file...). Both concepts are not really specific to hit testing. I added a general comment to the file.

It feels weird that this does the hit testing -- and that this class is now both the result and an object that performs hit testing.

It almost feels like we need a level of indirection here, with a notion of a HitTester object (that may have a transformation applied to it) to which the render box delegates.

But take with comment with a huge grain of salt, since it's a drive-by code review from someone who knows these innards much less than you do 😄

This does not perform the hit testing. It only transforms the coordinates to the coordinate space of the child, records the necessary transform and then executes the provided callback, which is supposed to implement the actual hit testing on the child. I updated the doc comment to make this clear.