Index Signature Parameter of type symbol

[kitsonk]

It would seem like the following should be valid:

interface A {
  [a: symbol]: any;
}

interface B {
  [a: symbol | string]: any;
}

But it errors with An index signature parameter type must be 'string' or 'number'.

[mhegazy]

Symbol indexers was not added along with other symbol support. here is the issue tracking adding it: #1863