threads in Legacy (Safari 7) browser

Hey, Im trying to use threads (1.7.0) in a legacy device - Safari 7
The code works on modern GoogleChrome, but when trying to run it on Safari 7, it throws when importing the library.

The following code prints only `Before load threads` and then throws `Super expression must either be null or a function`

```
try {
console.error('Before load threads');
const { Worker } = await import('threads');
console.error('after load threads'); // never prints
} catch (error) {
console.error('Error loading threads', error); // throws ...."Super expression must either be null or a function"
}
```
Any idea what can it be or how to fix it?

PS: The issue is not the async import, for sure: the file where this code is written, is itself imported async. And just to be sure I tried also with regular import (at the head of the file), and got the same error.

**Update 1**: The same happens with Safari 7, 8 and 9. It works on Safari 10

**Update 2**: The cause for the exception is in the file `master/implementation.browser.js`, it is checking ` if (typeof Worker === "undefined" `.
For Safari 3 and below, `typeof Worker === "undefined". For Safari 10 and up, `typeof Worker === "function"`.
But for Safari 7-9, `typeof Worker === "object". So when it tries to extend from Worker on line 24 `class WebWorker extends Worker ` it fails because is an object, not a function and throws the `Super expression ...` exception