Move Dialog to ES6

[samreid]

As a prelude to further work in #369, I'd like to see Dialog in ES6. I'll do that as a preliminary step.

UPDATE: There are 15 occurrences of Dialog.call( this, ... ), so we won't be able to use class until those cases are changed as well. According to the conversation in phetsims/phet-info#89 it is acceptable to port everything to ES6 except for the inherit part.

[samreid]

Not sure what else can be done aside from class/extends. @zepumph can you please take a look?

[zepumph]

• One potential improvement would be to make the internal type a class, but that may be confusing.

• I also prefer curly braces for scope if the arrow function is more than a one-liner. What do you think about the above commit?

• I removed function keyword from inherit call methods, how does that seem to you?

[samreid]

One potential improvement would be to make the internal type a class, but that may be confusing.

I think function CloseButton( options ) { should be deleted and replaced with usage of scenery-phet/js/buttons/CloseButton.js. If you agree, let's open a new issue for that.

I also prefer curly braces for scope if the arrow function is more than a one-liner. What do you think about the above commit?

No strong preference.

I removed function keyword from inherit call methods, how does that seem to you?

Looks great!

[zepumph]

I created #486. Closing