Fix error reporting for System.getProperty magic method nodes

[niloc132]

An earlier iteration appeared to add getProperty to GWT.java instead of using System. In that implementation, the "answers" for rebind seemed to be reused to hold default expressions, but that also changed before it was submitted to GWT.

Fixes #9806

[jnehlmeier]

Looks good, but please squash all commits before merging.

[niloc132]

Looks good, but please squash all commits before merging.

Thanks, standard process is as with gerrit, we always do squash-and-merge, with a single descriptive commit message.