Removes JavaScriptException dependency from c.g.gwt.core.shared.

Note that, this change also makes isClassMetadataEnabled public in the emulated version
of the java/lang/Class so that the logic for resolving types for the no metadata case
can be compiled out.

Change-Id: Ib610bb27cefb3d405e2126417008f4134743e723
Review-Link: https://gwt-review.googlesource.com/#/c/2840/