Use rollup-plugin-buble and use shorthand ES6 class methods

[IvanSanchez]

This uses Rich Harris' Bublé in order to use "Object shorthand methods and properties", so instead of

var foo = L.Class.extend({
    method: function(par1, par2) {
         ...
    }
});

One can write:

var foo = L.Class.extend({
    method(par1, par2) {
         ...
    }
});

...thus saving a few bytes on the ES6 code.

We might also want to use things like default values for function parameters, arrow functions, and other things that transpile well.

cc @mourner

[IvanSanchez]

Note this targets the rollup2 branch, not master

[mourner]

Buble is pretty cool! I don't see why we wouldn't want to use it (it's pretty much free if we're already using rollup), but I'd probably want to hold on upgrading to ES6-isms until we land ES6 modules to avoid merge complications.

[IvanSanchez]

Sounds like a sane thing to do. I'll close down this PR, but let's revisit this branch when #4989 is done.