dummypromise.js

// BEFORE
mw.flow.ui.EditorSwitcherWidget.prototype.switchEditor = function ( name ) {
  var widget = this;
  return this.convertContent( ... )
    .then( function ( newContent ) {
      return newEditor.setup( newContent );
    } )
    .then( function () {
      ...
    } )
    .then( function () {
      ...
    } );
};

// AFTER
mw.flow.ui.EditorSwitcherWidget.prototype.switchEditor = function ( name ) {
  if ( this.switchingPromise ) {
    return this.switchingPromise;
  }
  
  var dummy = $.Deferred(),
    widget = this;
  this.switchingPromise = dummy
    .then( function () {
      return widget.convertContent( ... );
    } )
    .then( function ( newContent ) {
      ...
    } )
    .then( function () {
      ...
    } )
    .then( function () {
      ...
    } );
  
  // NEW: emit event
  this.emit( 'switch', name, this.switchingPromise );
  dummy.resolve();
  return this.switchingPromise;
};