Implement Flow typechecking

[ivan-kleshnin]

As TypeScript is fundamentally incompatible with ES6 and Babel does not support it – it's not suitable for us. I'm going to check Flow. There will be a problems with IDEs, but let's try.

[ivan-kleshnin]

WebStorm does not officially support Flow for now
https://youtrack.jetbrains.com/issue/WEB-14254

But annotations for primitive types like

function foo(x: boolean, y: boolean): boolean {
  return x || y;
}

seems somehow compatible with IDE syntax expectations.
So the gradual introducing of Flow may be possible.

[ivan-kleshnin]

Incompatible with ES6:
facebook/flow#62

Suspended.

[ivan-kleshnin]

Checklist: facebook/flow#560

[ivan-kleshnin]

Not going to implement it in the foreseeable future. We have Tcomb which provides some benefits noone static typed platform gives. See https://twitter.com/IvanKleshnin/status/649135511584555008
Not going to trade them for another half-baked Facebooky stuff.