Union type inferring is somewhat incomplete

I've reproduced the bug using the online Flow type checker. Please refer to the following code:

```js
type TestType = {
type: "a",
a: number
} | {
type: "b",
b: number
};

const test = (a: TestType, b: TestType) => {
if (a.type === "a" && a.type === b.type) {
return b.a;
}
return 0;
}
```

This gives me an error:

```
11: return b.a;
^ Cannot get `b.a` because property `a` is missing in object type [1].
References:
9: const test = (a: TestType, b: TestType) => {
^ [1]
```

However, this gives no errors:

```js
type TestType = {
type: "a",
a: number
} | {
type: "b",
b: number
};

const test = (a: TestType, b: TestType) => {
if (a.type === "a" && b.type === "a" && a.type === b.type) {
return b.a;
}
return 0;
}
```

It should be inferred that because `a.type === "a" && a.type === b.type`, therefore `b.type === "a"`.