type refinement with typeof not working

[try flow](https://flow.org/try/#0PTAEAEDMBsHsHcBQAXAngBwKagIKgLygDeAPoqKAIYBcoAzsgE4CWAdgOYA05oAxrawCuAWwBGmRohIBfANyIUGbACECxMhVG0GLDtwoATASPGSZ8xVlyrCeEqGUXMAD3SxGyPrFYNQ7TMgAciYSagAUlFrWAJTGYqH4AHw8aFiwkFSiAHSUBPiEAOQ6bOwFoAD8mVm8oLSRWQayQA)

same example with same key but different type:
[try flow](https://flow.org/try/#0PTAEAEDMBsHsHcBQAXAngBwKagIKgLygDeAPoqKAIYBcoAzsgE4CWAdgOYA05oAxrawCuAWwBGmRohIBfANyIUGbACECxMhRqghYidwoATASPGSZ8xVlyrCeEqGUXMAD3SxGyPrFYNQ7TMgAciYSagAUlKK0OMoAlMa6jAQAfDxoWLCQVKIAdJQE+IQA5AwsHEWgAPzZObygtJE5BrJAA)

following code:

```
// @flow
type A = {|
a: string,
c: number
|};

type B = {|
b: string,
d: number
|};

type AB = A | B;

export const getNumber = (ab: AB): number =>
typeof ab.a === 'string' ? ab.c : ab.d;
```

gives following error:

```
15: typeof ab.a === 'string' ? ab.c : ab.d;
^ Cannot get `ab.c` because property `c` is missing in `B` [1].
References:
14: export const getNumber = (ab: AB): number =>
^ [1]
15: typeof ab.a === 'string' ? ab.c : ab.d;
^ Cannot get `ab.d` because property `d` is missing in `A` [1].
References:
14: export const getNumber = (ab: AB): number =>
^ [1]
```

It's clear that when `ab.a` is a string, it should have property `c`, and otherwise it should have property `d`.

Anyhow, with string literals the refinement is working: [try flow](https://flow.org/try/#0PTAEAEDMBsHsHcBQAXAngBwKagIKgLygDeAPoqKGlgFygDkAhnQDTmgO0DOyATgJYA7AOasKAY1oCArgFsARph6ISAXwDciFBmwAhAsTIUqmWnTks2HUNPmLRoACaTZCpao1asuPYTwlQOh6YAB7osDzIoGKwAtygQpjIAHIuivoAFAxytDg6AJTOtjwEAHyWcgB0xgT4hIx0oAD87JVioLRZFQ5qQA)

This is something that people expects flow to shine, but when it's not working it causes easily great frustration.