Reduce usage of heavy polyfills

[dantman]

core-js and regenerator are fairly heavy polyfills. At this point they probably aren't completely necessary. We should see if it's possible to eliminate as much of the dependency size as possible.

Perhaps raising the minimum Node.js version for 3.0.0 to Node.js 6.13, as soon as 4 goes EOL in April would be enough to eliminate most of the necessary transpiling done by regenerator.