getMapping = function(cssName) {
    return goog.cssNameMapping_[cssName] || cssName;
  }
Handles strings that are intended to be used as CSS class names.

This function works in tandem with @see goog.setCssNameMapping.

Without any mapping set, the arguments are simple joined with a
hyphen and passed through unaltered.

When there is a mapping, there are two possible styles in which
these mappings are used. In the BY_PART style, each part (i.e. in
between hyphens) of the passed in css name is rewritten according
to the map. In the BY_WHOLE style, the full css name is looked up in
the map directly. If a rewrite is not specified by the map, the
compiler will output a warning.

When the mapping is passed to the compiler, it will replace calls
to goog.getCssName with the strings from the mapping, e.g.
    var x = goog.getCssName('foo');
    var y = goog.getCssName(this.baseClass, 'active');
 becomes:
    var x= 'foo';
    var y = this.baseClass + '-active';

If one argument is passed it will be processed, if two are passed
only the modifier will be processed, as it is assumed the first
argument was generated as a result of calling goog.getCssName.

@param {string} className The class name.
@param {string=} opt_modifier A modifier to be appended to the class name.
@return {string} The class name or the concatenation of the class name and
    the modifier.