Support 'new.target' meta object

[DanielRosenwasser]

There isn't a lot in the current ES6 spec draft, but it seems to indicate the constructor object that is in the process of being new'd at runtime.

[rbuckton]

We also need to update es6.d.ts for Reflect.construct which has the signature:

export function construct(target: any, argArray: any[], newTarget?: any): any;

Where newTarget is the value for new.target.