Please try:

public class MediaTest {
    
    public static void main(String[] args) throws Exception {
        Class.forName("javafx.scene.media.MediaView").getDeclaredMethod("setViewport", javafx.geometry.Rectangle2D.class);
    }
}

=>

Exception in thread "main" java.lang.NoClassDefFoundError: com/sun/media/jfxmediaimpl/platform/ios/IOSMediaPlayer
at java.lang.Class.getDeclaredMethods0(java.base@9-ea/Native Method)
at java.lang.Class.privateGetDeclaredMethods(java.base@9-ea/Class.java:2952)
at java.lang.Class.getDeclaredMethod(java.base@9-ea/Class.java:2320)
at MediaTest.main(MediaTest.java:6)
Caused by: java.lang.ClassNotFoundException: com.sun.media.jfxmediaimpl.platform.ios.IOSMediaPlayer
at jdk.internal.loader.BuiltinClassLoader.loadClass(java.base@9-ea/BuiltinClassLoader.java:366)
at java.lang.ClassLoader.loadClass(java.base@9-ea/ClassLoader.java:419)
... 4 more

OS: Windows 7, Ubuntu 16.04 Linux. JDK9 b133

regression: no such issue for JDK 8. (jigsaw-related issue?)