Parse grouped ES6 fat-arrow expressions

Previously, grouped fat-arrow expressions ( () => assignmentExpr ) would yield an error. Now, they are parsed correctly. This CL also adds some additional fat-arrow test cases which demonstrate returning an object literal in concise function syntax.

Fixes #1692
The `moz` flag is for JavaScript 1.7, 1.8 features

- https://developer.mozilla.org/en-US/docs/Web/JavaScript/New_in_JavaScript/1.7
- https://developer.mozilla.org/en-US/docs/Web/JavaScript/New_in_JavaScript/1.8

These are SpiderMonkey-specific extensions to JS. The fat arrow should be allowed only for the `esnext` flag