code: function openPanel(preset, opt_config) {
      // TODO(ErinCoughlan): Investigate why panelRef.open() doesn't return
      // panelRef.
      var openPromise;
  
      if (panelRef) {
        openPromise = panelRef.open();
      } else {
        openPromise = $mdPanel.open(preset, opt_config);
      }
  
      openPromise.then(function(createdPanelRef) {
        panelRef = createdPanelRef;
        return panelRef;
      });
  
      flushPanel();
    }
docs: Opens the panel. If a config value is passed, creates a new panelRef
  using $mdPanel.open(config); Otherwise, called open on the panelRef,
  assuming one has already been created.
  @param {!Object=} opt_config