Allow string literal types as index

[scharf]

TypeScript Version:

1.8.2

Code

type FooBar = "foo" | "bar";

var map:{[foobBar:FooBar]: boolean};

Expected behavior:

I should be able to define a index type with a string literal type as key.

Actual behavior:

The typescript compiler complains with

Error:(3, 11) TS1023: An index signature parameter type must be 'string' or 'number'.

[mhegazy]

looks like a duplicate of #6246

[mhegazy]

actually #5683 is a better one to reference.