[Feature request] const's type should using it's value

[zaaack]

e.g.
current:

const white = '#fff'
// typeof white = string

expect:

const white = '#fff'
// typeof white = '#fff'

So in nuclide autocomplete can show the value of consts, just like VS Code.

[0rvar]

I think this is a good suggestion. A literal string type is a subtype of string anyway.

[agentcooper]

Duplicate of #2639.