Extends expression performs differently on Edge

[Gareth Heyes]

```javascript
class y{}class z{}
class x extends y, z{}/alert(1)/+alert(2)
```

Edge seems to allow non-standard syntax here. I guess to allow you to
extend multiple classes but as far as I'm aware this is non-standard
syntax. The code above calls alert(2), if you change it to a class
expression both alerts are called. It's a syntax error on other browsers.
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