tsconfig should use lib: [ 'es2015', 'dom' ] instead of @types/core-js

[robwormald]

Using core-js types (the polyfill itself is fine) leads to conflicts between native promises and core-js's version.

[filipesilva]

Will work on it. We also need to do it on the docs.

[wardbell]

Slated for QuickStart Reboot PR #287