Revert "instrument Display.focusProperty for all a11y sims, not just …

…based on query parameter, #936"

This reverts commit ab99d0f.

js/display/Display.js

@@ -274,20 +274,6 @@ define( function( require ) {
     this.scenery = scenery;
 
     if ( this.options.accessibility ) {
-
-      // If the display supports accessibility, we want to overwrite the focusProperty, instead instrumenting it with
-      // PhET-iO, see https://github.com/phetsims/scenery/issues/936
-      // NOTE: Do not move this below FocusOverlay's creation below.
-      Display.focusProperty = new Property( null, {
-
-        // Make this a static tandem so that it can be added to PhET-iO Studio correctly (batched and then flushed when the
-        // listener is added).
-        tandem: Tandem.generalTandem.createTandem( 'focusProperty' ),
-        phetioType: PropertyIO( NullableIO( FocusIO ) ),
-        phetioState: false,
-        phetioReadOnly: true
-      } );
-
       if ( this.options.isApplication ) {
         this._domElement.setAttribute( 'aria-role', 'application' );
       }
@@ -1795,12 +1781,23 @@ define( function( require ) {
     'scenery-grabbing-pointer': [ 'grabbing', '-moz-grabbing', '-webkit-grabbing', 'pointer' ]
   };
 
-  // @public (a11y, read-only, scenery-internal setable) {Property.<Focus|null>} - Display has an axon Property to
-  // indicate which component is focused (or null if no scenery node has focus).
-  // NOTE: DO NOT LINK TO THIS!!! It is potentially overridden in Display's constructor
-  // to support PhET-iO Instrumentation.  see https://github.com/phetsims/scenery/issues/936
-  // FocusOverlay links to this as an exception, but only after it is overwritten, see Display's constructor.
-  Display.focusProperty = new Property( null );
+  // @public (a11y, read-only, scenery-internal setable) {Property.<Focus|null>} - Display has an axon Property to indicate which
+  // component is focused (or null if no scenery node has focus).  By passing the tandem and phetioValueType,
+  // PhET-iO is able to interoperate (save, restore, control, observe what is currently focused. See Display.focus
+  // for setting the Display's focus. Don't set the value of this Property directly.
+  Display.focusProperty = new Property( null,
+
+    // Only instrument if accessibility is enabled
+    ( window.phet && phet.chipper && phet.chipper.accessibility ) ? {
+
+      // Make this a static tandem so that it can be added to PhET-iO Studio correctly (batched and then flushed when the
+      // listener is added).
+      tandem: Tandem.generalTandem.createTandem( 'focusProperty' ),
+      phetioType: PropertyIO( NullableIO( FocusIO ) ),
+      phetioState: false,
+      phetioReadOnly: true
+    } : {}
+  );
 
   // @public {Emitter} - Fires when we detect an input event that would be considered a "user gesture" by Chrome, so
   // that we can trigger browser actions that are only allowed as a result.