Support spreading and destructuring of iterators in ES6

[JsonFreeman]

This adds support for a few things:

• Spreading an iterator into an array

var array = [0, 1, ...new NumberIterator];

• Spreading an iterator into a call

function foo(...nums: number[]) { }
foo(...new NumberIterator);

• Destructuring an iterator into an array

var [a, b, ...c] = new NumberIterator; // a and b have type number, c has type number[]
var d: number, e: number, f: number[];
[d, e, ...f] = new NumberIterator;
function foo([p, q, ...r]: Iterable<number>) {}
foo(new NumberIterator);

I will also add that tuples are relatively unaffected. If you declare something to be a tuple, you can still only assign a tuple to it. And if you have an array binding pattern with no rest element, its implied type is a tuple. So the following does give you an error:

function foo([p, q]) {}
foo(new NumberIterator);

[CyrusNajmabadi]

That's an odd requirement given that the grammar only says this:

RestParameter:
    ...   Identifier   TypeAnnotation_opt

I don't see why, at grammar check time, we'd care if the parameter was an array type or not.

[JsonFreeman]

It's the type system, not the grammar checker. I will fix the comment

[JsonFreeman]

To see a design proposal, see #1900 (comment)

[yuit]

forgot to approve 👍