Intersection of object types should be a valid lower bound of the object type with properties from all branches

[samwgoldman]

Repro:

/* @flow */
type A = { a: string } & { b: string };
type B = { a: string, b: string };
declare var a: A;
var b: B = a;

test.js:5
  5: var b: B = a;
                ^ intersection: object type(s). This type is incompatible with
  5: var b: B = a;
            ^ object type

What's happening here, as far as I can tell, is that Flow tries both branches of the intersection in A one at a time. As if we assigned var b: B = { a: string } then later var b: B = { b: string }. Both of these assignments independently should fail, but if we imagined that both assignments happened "together" should succeed.

I think that lots of issues related to intersection types might reduce to this. Check out linked issues for more examples.

[pasha-mf]

Seems like #1172 is related.