Remove dependency on object-assign

[Chalarangelo]

As the code gets processed anyways, the dependency on a polyfill such as object-assign is not necessary. I replaced said dependency with an assignment to the native Object.assign method, so that the code works exactly like before but with one less dependency.

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[ljharb]

If the negligible cost of a single dependency is a problem, you can alias it out in your bundler.

[jimmywarting]

What if we changed the only line it's used? from

var allKeys = assign({}, props[propName], shapeTypes);
for (var key in allKeys) {

to

var allKeys = Object.keys(props[propName]).concat(Object.keys(shapeTypes));
for (var i = 0; i < allKeys.length; i++) {
  var key = allKeys[i]

Object.keys is supported in more browsers

[jimmywarting]

Another tip since you have babel

var allKeys = assign({...props[propName], ...shapeTypes});
for (var key in allKeys) {

[ljharb]

That would be different; Object.keys doesn’t include symbols, and neither does for..in.