keyof of 'mixed' class equals to `string | number`

TypeScript Version: 3.4.3 or 3.5.0-dev.20190427

Search Terms: keyof, anonymous class

Code

type AnyClass = new (...args: any[]) => any;

const mixin = <T extends AnyClass>(Class: T) => class extends Class {a: string};
const Cl = mixin(class {b = 123});
const inst = new Cl();

inst. //  <- IntelliSense works fine here and suggests 'a' and 'b' properties

type Keys = keyof typeof inst; // Keys = string | number

Expected behavior:
Keys to be equal to 'a' | 'b'

Actual behavior:
Keys to be equal to string | number

Playground Link: ts-playground

Description:
I'm sorry if that is a known bug ot expected behaviour. I did not find a duplicate.
Class C is a result of extension of some other class A provided as a function argument and class B which is declared inside the function body. Such class C works fine for the most part, but keyof InstanceType<typeof C> returns string | number.

While IntelliSense works fine:

I'll be glad if you suggest any workarounds for this issue.

Start by reproducing the example in the linked TypeScript Playground using the reported versions and confirm the inferred type of keyof typeof inst. Trace the compiler's handling of keyof for the mixed anonymous class and add coverage for the example so that the result is 'a' | 'b' rather than string | number.