fix for #64

[doomb0t]

This is my attempt to implement the optimizations mentioned in the issue. I assumed it was also safe to remove the 'in' check from 'getPositiveSpacing' as well because if it is null then will not be >= 0 I would be happy to make any requested changes if this isn't quite right.
Thanks!

[vjeux]

this is not safe :(

var node = { style: { marginTop: 0, margin: 10 } };

// before
if ('marginTop' in node.style) {
  return node.style['marginTop'];
}
// returns 0

// after
return node.style['marginTop'] || node.style['margin'];
// returns 10

[vjeux]

A safe way to do it is by writing node.style['marginTop'] !== undefined. But, in order to make sure that it has performance improvement, we need to benchmark it.

Can you use the code that generates a random test to generate a big random tree and then time the layout algorithm before and after? Otherwise we can't know if this is a performance optimization or not.

[doomb0t]

Hey,
I will gladly make this safer then time the tests and give you the output. I should have thought out my changes a bit more thoroughly, sorry about that!

[doomb0t]

Also would making it
return node.style['marginTop'] || node.style['margin'] || 0;
work if I excluded the return 0 on exiting the loop? Otherwise I can add check for !== undefined.
Also for the tests do I use the RunLayoutRandomTests.html in chrome to do this?

[doomb0t]

Hey so after working on this for a while yesterday I found that there wasn't much of a performance improvement on my machine, and that the changes caused a slight increase in the number of failures on the layout tests. Thanks for taking a look at what I had. I will close the PR.