[3.5.1] Class index signature stopped working - property does not exist on type

[aszczepanek]

TypeScript Version: 3.5.1

Search Terms:
index signature, property does not exist

Code

export class Test {
  [k: string]: any;

  someMethod() {
    this.age = 3;
  }
}

Expected behavior:
There should be no error, age assignment is correct according to index signature.
It used to work in typescript 3.4.5

Actual behavior:
Error "Property 'age' does not exist on type 'Test'"

[ahejlsberg]

Argh, I recently fixed this in #31454, but neglected to update the separate code path we have for property access expressions.