Class DictEmitter

UserDict.UserDict --+        
                    |        
     Util.OrderedDict --+    
                        |    
            Util.Selector --+
                            |
                           DictEmitter

A callable dictionary that maps file suffixes to emitters.
When called, it finds the right emitter in its dictionary for the
suffix of the first source file, and calls that emitter to get the
right lists of targets and sources to return.  If there's no emitter
for the suffix in its dictionary, the original target and source are
returned.