Classes with symbols don't pass validation in strict mode

[amir-arad]

TypeScript Version: 2.3.3

Code
(copy-paste from the symbols documentation)

const getClassNameSymbol = Symbol();

class C {
    [getClassNameSymbol](){
       return "C";
    }
}

let c = new C();
let className = c[getClassNameSymbol](); // Error TS7017

Expected behavior:
pass walidation
Actual behavior:
TS7017:Element implicitly has an 'any' type because type 'C' has no index signature.

[ghost]

May be fixed by #15473? @rbuckton

[mhegazy]

Duplicate of #5579

[mhegazy]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.