Cannot coerce symbol (from 5th property on BigInt64Array) to string


```js
const monster = Reflect.ownKeys(BigInt64Array);
const sonOfMonster = Object(monster[4]);

// these don't throw.
String(Symbol('hello'));
String(Symbol());

String(sonOfMonster);
// TypeError: String: Cannot coerce symbol to string
```


**Build environment:** Linux
**Target device:** xsnap

cc @warner @erights @kriskowal @phoddie