Use goog.dom.classlist functions

The goog.dom.classes functions have now been deprecated.  See https://github.com/google/closure-library/commit/97e8a0c0fc7238a56cc4dacd4a96fd4c0735b992
Merge pull request #2284 from tschaub/classes

Use `goog.dom.classlist` functions instead of the deprecated `goog.dom.classes` functions.