Clean up.

src/cameras/OrthographicCamera.js

@@ -7,6 +7,8 @@ class OrthographicCamera extends Camera {
 
 		super();
 
+		Object.defineProperty( this, 'isOrthographicCamera', { value: true } );
+
 		this.type = 'OrthographicCamera';
 
 		this.zoom = 1;
@@ -132,6 +134,4 @@ class OrthographicCamera extends Camera {
 
 }
 
-OrthographicCamera.prototype.isOrthographicCamera = true;
-
 export { OrthographicCamera };