cast and accessor with target es6 produces invalid code

[MrAndersen1]

TypeScript Version: 2.3.0-dev.20170328

Code

The following code produces wrong output, when run through tsc, only when target is es6, the "as"-cast exists and the accessor "[x]" exists.

(x) => ({ "1": "one", "2": "two" } as { [key: string]: string })[x];

Expected behavior:
Running "tsc -t es6" on the above code in a .ts file I expect to get a .js file containing:
(x) => ({ "1": "one", "2": "two" })[x];

Actual behavior:
(x) => { "1": "one", "2": "two" }[x];