(which is good: our tests have intentional violations), but
                     Error Prone doesn't know it's building test code unless we
                     pass -XepCompilingTestOnlyCode, and that argument needs to
                     be passed as part of the same <arg> as -Xplugin:ErrorProne,
                     and I gave up trying to figure out how to do that for test

            Let's take the warnings for one module, c.g.c.base.testModule, as an example:

              [INFO] Validating units:
              [INFO]    Ignored 53 units with compilation errors in first pass.
              [INFO] Compile with -strict or with -logLevel set to TRACE or DEBUG to see all errors.

            Turning up the log level, we see that GWT is trying to compile classes from other