`qx.test.core.Validation: warning: The mangling of private variable '__validateCustom' has been blocked because it is referenced as a string before it is declared`

As in the case of #821, i got a warning message from the compilation with typescript (`qx compile --typescript`)

```
qx.test.core.Validation: warning: The mangling of private variable '__validateCustom' has been blocked because it is referenced as a string before it is declared
```

thanks.