ClassLoader::getSystemClassLoader will never be null

JDK 9 defines ClassLoader::getPlatformClassLoader [1] that is parent or ancestor of the system class loader [1]. The system class loader will never be null.

ClassLoader::getSystemClassLoader spec should be updated that the returned value is non-null.

[1] http://download.java.net/java/jdk9/docs/api/java/lang/ClassLoader.html#builtinLoaders