Add gwt-core dependency for GWT.isProdMode()

[niloc132]

This check lets us add some checks in dev mode, instead of reading the property, since that would conflict with gwt-core in a project that uses both.

[FrankHossfeld]

Is only needed inside the GWT test module. I think, we can close the issue.

[niloc132]

Probably should be fixed though, in case a later test ends up depending on gwt-core, or some later feature depends on gwt-core.