Cannot implement Iterable interface

If I implement an `Iterator` interface with `Symbol.iterator` like this, it works at runtime.

```js
let iterable = {
[Symbol.iterator]: function*() {
yield 1;
yield 2;
yield 3;
},
};
console.log([...iterable]);
```
```
[ 1, 2, 3 ]
```

But Flow complains that the object isn't iterable, expecting a property named `"@@iterator"`:

```
8: console.log([...iterable]);
^ object literal. This type is incompatible with
8: console.log([...iterable]);
^ $Iterable
Property `@@iterator` is incompatible:
8: console.log([...iterable]);
^ property `@@iterator` of $Iterable. Property not found in
8: console.log([...iterable]);
^ object literal
```

If I change the name of the property to `"@@iterator"` like this, Flow is happy.

```js
let iterable = {
"@@iterator": function*() {
yield 1;
yield 2;
yield 3;
},
};
console.log([...iterable]);
```
```
No errors!
```

But it fails at runtime:

```
TypeError: iterable[Symbol.iterator] is not a function
```

I can't keep the runtime and the typechecker happy at the same time.