allow JsObject in toDart/toJs

This allows to retrieve anonymous js object returned by a method. `registerFactoryForJsConstructor` cannot be used in that case.
This also can be used as a workaround to use List in parameters (once `@JsName` has landed).

I submitted the same PR ( #165 ) but it seems to have been destroyed by the branch renaming.

If the close was wanted, could you tell me why please ?