Fix some of the warning messages produced at verbose warning level

When Closure Compiler is run with --warning_level=verbose, there are many warnings and errors produced by the annotations.  This pull request fixes some, but not all, of those warnings in the core modules.

If you are interested in supporting verbose mode, you may wish to consider adding `warning_level: "VERBOSE"` to `tests/validate.js`.  I'm sure some of your users would find it as useful as I do.

Thanks!