Steps:
1. Download and unzip the GWT SDK zip
2. Create a directory for the GWT version being added, and cd into it
3. Copy gwt-dev.jar and gwt-user.jar into this new directory
4. Run ./createApiCheckerReferenceJars.sh
   - Ignore the benign error about finding com/google/web.  The directory only exists in gwt-user.
5. Delete gwt-dev.jar and gwt-user.jar from this directory
6. Commit the new gwt-dev-modified.jar and gwt-user-modified.jar