Fix conflict between constant and style class name.

In GWT-CSS, a constant and a style class can share the same name
if in the interface there is a method for the constant that doesn't
return String and another method for the style class using
the @ClassName annotation.

Change-Id: I7406e053d121d6e821322bd50428f58feedba9ba
(cherry picked from commit 139e7f0002ad8a7b193864682fe85fc462dd9606)