Legacy classes TwoStacksPlainDatagramSocketImpl and TwoStacksPlainSocketImpl were removed from JDK code base in JDK-8201510 and JDK-8253119. However, there are still some references to the removed classes in test code:
test/jdk/java/net/DatagramSocket/UnreferencedDatagramSockets.java
test/jdk/java/net/MulticastSocket/UnreferencedMulticastSockets.java
test/jdk/java/net/ServerSocket/UnreferencedSockets.java

They should be cleaned up.