Use property types to infer accessor types

[Simn]

see #10568

I kinda hate how I implemented this, but at least we have a test case now.

[Simn]

There's some null-safety failure:

This is now typed as set_str(v:String):String. I think the test relied on v being inferred to Null<String>, but in that case we should just type-hint it directly.