_unsafeReadProtoTagged should not rely on window

Current implementation of `_unsafeReadProtoTagged` relies on the window object
```
if (typeof window !== "undefined") {
    var ty = window[name];
    if (ty != null && value instanceof ty) {
      return just(value);
    }
```
But it's possible that the value or event comes from a different window context, like an event from iframe, then `value instanceof ty` will fail.