function handleProgress() {
                        
      var textSoFar = xhr.responseText,
          newText = textSoFar.substr(numberOfCharsAlreadyGivenToCallback);
      
      
      /* Raise the event for new text.
      
         On older browsers, the new text is the whole response. 
         On newer/better ones, the fragment part that we got since 
         last progress. */
         
      if( newText ) {
         emitStreamData( newText );
      } 

      numberOfCharsAlreadyGivenToCallback = len(textSoFar);
   }
Handle input from the underlying xhr: either a state change,
the progress event or the request being complete.