old_contents: // Copyright 2019, University of Colorado Boulder
  
  /**
   * Button group that contains a play/pause and step button.
   *
   * @author Chris Klusendorf (PhET Interactive Simulations)
   */
  define( function( require ) {
    'use strict';
  
    // modules
    var EFACConstants = require( 'ENERGY_FORMS_AND_CHANGES/common/EFACConstants' );
    var energyFormsAndChanges = require( 'ENERGY_FORMS_AND_CHANGES/energyFormsAndChanges' );
    var HBox = require( 'SCENERY/nodes/HBox' );
    var inherit = require( 'PHET_CORE/inherit' );
    var PlayPauseButton = require( 'SCENERY_PHET/buttons/PlayPauseButton' );
    var StepForwardButton = require( 'SCENERY_PHET/buttons/StepForwardButton' );
  
    /**
     * @param {EFACIntroModel|SystemsModel} model
     * @constructor
     */
    function PlayPauseStepButtonGroup( model ) {
  
      // create the play/pause button
      var playPauseButton = new PlayPauseButton( model.isPlayingProperty, {
        radius: EFACConstants.PLAY_PAUSE_BUTTON_RADIUS
      } );
  
      // create the step button, which is used to manually step the simulation
      var stepButton = new StepForwardButton( {
        isPlayingProperty: model.isPlayingProperty,
        listener: function() { model.manualStep(); },
        radius: EFACConstants.STEP_FORWARD_BUTTON_RADIUS
      } );
  
      HBox.call( this, {
        children: [ playPauseButton, stepButton ],
        spacing: 10
      } );
    }
  
    energyFormsAndChanges.register( 'PlayPauseStepButtonGroup', PlayPauseStepButtonGroup );
  
    return inherit( HBox, PlayPauseStepButtonGroup );
  } );
  
  
new_contents: // Copyright 2019, University of Colorado Boulder
  
  /**
   * Button group that contains a play/pause and step button.
   *
   * @author Chris Klusendorf (PhET Interactive Simulations)
   */
  define( require => {
    'use strict';
  
    // modules
    const EFACConstants = require( 'ENERGY_FORMS_AND_CHANGES/common/EFACConstants' );
    const energyFormsAndChanges = require( 'ENERGY_FORMS_AND_CHANGES/energyFormsAndChanges' );
    const HBox = require( 'SCENERY/nodes/HBox' );
    const PlayPauseButton = require( 'SCENERY_PHET/buttons/PlayPauseButton' );
    const StepForwardButton = require( 'SCENERY_PHET/buttons/StepForwardButton' );
  
    class PlayPauseStepButtonGroup extends HBox {
  
      /**
       * @param {EFACIntroModel|SystemsModel} model
       */
      constructor( model ) {
  
        // create the play/pause button
        const playPauseButton = new PlayPauseButton( model.isPlayingProperty, {
          radius: EFACConstants.PLAY_PAUSE_BUTTON_RADIUS
        } );
  
        // create the step button, which is used to manually step the simulation
        const stepButton = new StepForwardButton( {
          isPlayingProperty: model.isPlayingProperty,
          listener: () => model.manualStep(),
          radius: EFACConstants.STEP_FORWARD_BUTTON_RADIUS
        } );
  
        super( {
          children: [ playPauseButton, stepButton ],
          spacing: 10
        } );
      }
    }
  
    return energyFormsAndChanges.register( 'PlayPauseStepButtonGroup', PlayPauseStepButtonGroup );
  } );
  
  
subject: Convert to ES6, see https://github.com/phetsims/energy-forms-and-changes/issues/211