Child properties do not properly influence parent object's type

If you alias an object's field (or simply deconstruct it) and write a guard statement that specifies a property of the obtained value, original object's type does not reflect the newly discovered properties.

Consider the following:

``` js
type Foo = {a: '1', b: any} | {a: '2'}

function test(foo: Foo) {
const { a } = foo // deconstruction
switch (a) {
case '1':
// at this point Flow knows what `a` is but not what `foo.a` is
console.log(foo.b) // this fails
break
}
if (a === '1') {
// at this point Flow knows what `a` is but not what `foo.a` is
console.log(foo.b) // this fails
}
}
```

However this works as expected:

``` js
type Foo = {a: '1', b: any} | {a: '2'}

function test(foo: Foo) {
switch (foo.a) {
case '1':
console.log(foo.b) // this works
break
}
if (foo.a === '1') {
console.log(foo.b) // this works
}
}
```

Seeing that `a` was not mutated between the calls, its type properties should also affect its parent's properties within the scope of the guard (`if`, `switch/case` etc.).

Currently it's super inconvenient to write code as you have to work around this by always referring to the parent object.