fix(#2644): adapt the react children api

[deleonio]

Fixes #2644 .

[deleonio]

I still have to adjust the tests.

[deleonio]

I think that is relevant too ... https://reactjs.org/docs/react-api.html#reactchildren

[coveralls]

Coverage remained the same at 100.0% when pulling 8265ed8 on martinoppitz:fix/#2644 into 8603d70 on preactjs:master.

[deleonio]

I hadn't seen that. I feel like a beginner.

[marvinhagemeister]

No worries, like mentioned on twitter the this argument combined with Function.prototype.call() and Function.prototype.apply() is pretty esoteric these days and hardly used much anymore. So it's easy to miss 👍

[marvinhagemeister]

This is a great start, but I think the commit isn't complete. The test defines a few functions but never calls them anywhere. This will make the test always pass, even against current master and without the changes in this PR.

Suggested change

	it('should propagate the this context', () => {
	it('should propagate this context', () => {
	const context = {};
	const spy = sinon.spy(child => child); // noop
	const Foo = ({ children }) => {
	return React.Children.map(children, spy, context);
	};
	render(<Foo>foo</Foo>, scratch);
	expect(spy.thisValues[0]).to.equal(context);
	});

[developit]

We don't generally use reduce(). I'd prefer to use .map(fn.bind(ctx)) for size and perf reasons.