Assign function instead of MDN polyfill

assign.js

/**
 * Copy the values of all enumerable own properties from one or more objects to a target object.
 * Portion taken from the Object.assign(target, ...sources) polyfill on MDN.
 * @see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/assign#Polyfill
 * @param {object} target Target object
 * @param {...object} sources Additional objects whose properties to copy into target
 * @return {object} target
 */
export default function assign(target, ...sources) {
  if (target == null) { // TypeError if undefined or null
    throw new TypeError('Cannot convert undefined or null to object');
  }

  var to = target;

  for (var index = 1; index < arguments.length; index++) {
    var nextSource = arguments[index];

    if (nextSource != null) { // Skip over if undefined or null
      for (var nextKey in nextSource) {
        // Avoid bugs when hasOwnProperty is shadowed
        if (Object.prototype.hasOwnProperty.call(nextSource, nextKey)) {
          to[nextKey] = nextSource[nextKey];
        }
      }
    }
  }
  return to;
};