Migration to TypeScript 2.3

[kitsonk]

This is a master issue to address migration to TypeScript 2.3 features that we care about:

• Migrate to --strict flag.
• Adopt this typing.
• Implement generators (now supported on ES3/ES5).
• Implement generic type defaults.

There is also basic plugin support for the language services in 2.3, I don't know if there is anything that would benefit from writing a plugin.

[kitsonk]

TypeScript 2.3 is Released