"new EventTarget()" or "extends EventTarget" throw errors

Closure compiler should allow instantiating or extending the native EventTarget class, see [1]. Also see [2] which depicts that this is already implemented in Chrome.

Unfortunately closure compiler throws an error for both these cases.
```js
const foo = new EventTarget();
class MyTarget extends EventTarget {}
```

```
JSC_NOT_A_CONSTRUCTOR: cannot instantiate non-constructor at line 1 character 12
const foo = new EventTarget();
^
JSC_CONFLICTING_EXTENDED_TYPE: MyTarget cannot extend this type; constructors can only extend constructors at line 2 character 0
class MyTarget extends EventTarget {}
^
```

Even if this is too hard to fix in the default JS compiler externs, there should be a command line flags that allows using a different set of externs to achieve this.

See previous PR about this at https://github.com/google/closure-compiler/pull/2988, which never landed.

[1] https://developer.mozilla.org/en-US/docs/Web/API/EventTarget/EventTarget
[2] https://www.chromestatus.com/features/5721972856061952