UserAgent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/67.0.3396.99 Safari/537.36

Steps to reproduce the problem:
1. Execute this code:
```js
function f() {for (b of null);};
for (i = 3; i--;) setTimeout(f);
```
2. Observe the error messages logged in console.
3. The first message will be `Uncaught TypeError: null is not iterable`, but the second (and third, and so-on) will be `Uncaught TypeError: Cannot read property 'Symbol(Symbol.iterator)' of null`.

What is the expected behavior?
The same error message should be returned for the same error message every time.

What went wrong?
A different error message is produced only for the first time an erroneous function is called. Further calls produce the other error message.

Did this work before? N/A 

Chrome version: 67.0.3396.99  Channel: stable
OS Version: 10.0
Flash Version: 

This is really minor, since there's nothing functionally wrong, but it did strike me as important enough to report, because it caused me to have to re-investigate several dozen lines of code due to believing I must be mutating state unintentionally somewhere to cause the same code to produce 2 different error messages.