No taint propagation through StringIterator

Given the following JavaScript code:
```js
[...String.tainted("abc")].map(x => x.taint)
```

Expected:
```
[
  [{ begin: 0, end: 1, flow: [...] }],
  [{ begin: 1, end: 2, flow: [...] }],
  [{ begin: 2, end: 3, flow: [...] }],
]
```

Actual:
```
[ [], [], [] ]
```