I think I found the issue that has been plaguing most of us when sometimes building the GWT coregui module.
If you recall, it seems as though whenever we pull from git any changes in coregui that a) add messages to Messages.properties or b) add to the RHQ server-side GWT RPC API, the coregui build fails and we have to do a mvn clean in coregui to get it to build again.
I think this is a side-effect of using gwt.draftCompile=true (which you probably have in your ~/.m2/settings.xml file). I think gwt.draftCompile=true skips the code generation step for the i18n message interface and the GWT Async interfaces.
If you get errors while doing a coregui build, try one of these two things - you have to either:
a) mvn clean before rebuilding coregui
or
b) use -Dgwt.draftCompile=false as a mvn command line option.
Note that turning off draftCompile actually DOUBLES my compilation time on my box (coregui goes from ~2 minutes build time to ~4 minutes).