Convert elements to use ES6 modules (import/export)

[etimberg]

Converts the element files (src/elements/*) to use import/export.

[etimberg]

I used a non default export here since I think that will allow something like

import { Arc } from 'elements';

[etimberg]

I started with the elements since I think the diff is minimal for now. The helpers require a lot more changes since we'd want to move toward exporting individual functions rather than exporting one large object.

[kurkle]

Great!
Just some wondering about style we should be using.

[kurkle]

Should we use the

export default class Line extends ...

style instead?

Thinking about style consistency when we get to helpers:

export function helper1() {};
export function helper2() {};

vs

function helper1() {}
function helper2() {}

export {
  helper1
  helper2
}

[etimberg]

I started playing with the helpers files. Personally I preferred the first style but I'm not opposed to the 2nd if that's what everyone else prefers

[kurkle]

I prefer brevity

[etimberg]

Not sure why 1 test is failing. It says that it failed to create the chart, but I don't know why. Will debug later today

[kurkle]

Chart namespace Chart.elements should be an object FAILED
Expected false to be truthy.

Not sure what instanceof those static import's are.

[etimberg]

Yeah, I moved to a default export and it worked. Tests are still failing until #6780 is merged

[benmccann]

I closed and reopened the PR to kick off Travis again. The tests are passing now