type check issue #16986

I know it's a tricky construction in begin with - but it was compiling fine before 2.4.

class Foo {
  [id: string]: { a?: number, b?: number };
  v1?: string;
}

with TSC 2.4.1, it gives me an error:
TS2411: Property 'v1' of type 'string' is not assignable to string index type '{ a?: number, b?: number }'.