[gltf] Handle r86-dev changes alongside backwards-compat.

examples/js/loaders/GLTF2Loader.js

@@ -2198,7 +2198,16 @@ THREE.GLTF2Loader = ( function () {
 
 						if ( geometry.attributes.normal === undefined ) {
 
-							material.shading = THREE.FlatShading;
+							if ( material.flatShading !== undefined ) {
+
+								material.flatShading = true;
+
+							} else {
+
+								// TODO: Remove this backwards-compatibility fix after r87 release.
+								material.shading = THREE.FlatShading;
+
+							}
 
 						}