Use @expose for Coordinate.z

By annotating the z property with @expose the compiler does not try to optimize that property in any way. See https://developers.google.com/closure/compiler/docs/js-for-compiler. The doc says that @expose should never be used in library code, but that's the only way I found for our case.
Merge pull request #435 from elemoine/coordinate-z

Use @expose for Coordinate.z