The class method census marks a method name as replaceable when a
function-typed property write uses that name, so a scanner storing its
error callback in `this.onError` made every call of any class's
`onError` method go through the checked property bag. That bag is a
dynamic store naming any property, so the int32 slot analysis also gave up
int32 storage for every class field exposed to dynamic code.
A function-typed write now marks the name only when an instance of a class
that has the method can inhabit the receiver's type: an assignability check
against the instance types of the method's owners, with `this` in an
instance member read as its class. Generic or anonymous owners, `this`
outside instance members, type parameters, `any` and `unknown` receivers
keep the conservative answer. Writes through interfaces or subclasses that
can reach a method still replace it.
Test: class-method-mutations.test.ts. Corpus:
4597-callback-field-method-name.
