anchor.diff:  /// The multiplexer is an intermediary that handles the routing of requests
   /// between caches and the true Apiary interface.
   class Multiplexer extends RequestHandler {
  +  
  +  static const AGE_NO_RPC = -1;
  +  static const AGE_CACHE_LOOKUP_ONCE = -2;
   
     /** Cache instance. */
     final Cache _cache;
  
positive.code: 
    Multiplexer(this._delegate, {Cache cache: null})
        : this._cache = cache == null ? new AsyncMapCache() : cache;
  
    Stream handle(Request request) {
      // Make a copy of the request for use in the multiplexer, since it's not
      // immutable.
      request = request.clone();
  
      // Create a new stream for this request.
      var active = new _ActiveStream(request);
      active.closed.whenComplete(() => _removeActive(active));
  
      // Only cachable requests need to be handled by the multiplexer (right now).
      if (request.isCachable) {
  
  
negative.code:         pending = _inFlightRequests[request];
        }
  
        // RPC replies are handled in one place, but errors for requests are
        // subscribed to individually. This is because only in-flight requests
        // should have error returns, whereas all streams for a request care when
  