No error for assignment to undefined inside extends expression

Hello,
The following code behaves strangely (inconsistent with other engines).
```
class cc extends ( x = function () {
console.log("Hi");
}) {
};
```

In Edge, cc is defined normally and x is defined too.
However, in V8 and Firefox, a ReferenceError is reported:
`ReferenceError: assignment to undeclared variable x`

BT group
2018.6.26