code: function() {
  
          var sinks = [];
          joint.util.forIn(this._nodes, function(exists, node) {
              if (!this._out[node] || joint.util.isEmpty(this._out[node])) {
                  sinks.push(this.getCell(node));
              }
          }.bind(this));
          return sinks;
      }
docs: Get all the leafs of the graph. Time complexity: O(|V|).