SPIGOT-1776: Replace BossBar.hide/show with visibility.

nms-patches/BossBattleServer.patch

@@ -0,0 +1,11 @@
+--- a/net/minecraft/server/BossBattleServer.java
++++ b/net/minecraft/server/BossBattleServer.java
+@@ -10,7 +10,7 @@
+ 
+     private final Set<EntityPlayer> h = Sets.newHashSet();
+     private final Set<EntityPlayer> i;
+-    private boolean j;
++    public boolean j; // PAIL: rename / public
+ 
+     public BossBattleServer(IChatBaseComponent ichatbasecomponent, BossBattle.BarColor bossbattle_barcolor, BossBattle.BarStyle bossbattle_barstyle) {
+         super(MathHelper.a(), ichatbasecomponent, bossbattle_barcolor, bossbattle_barstyle);

src/main/java/org/bukkit/craftbukkit/boss/CraftBossBar.java

@@ -146,6 +146,16 @@ public class CraftBossBar implements BossBar {
         return players.build();
     }
 
+    @Override
+    public void setVisible(boolean visible) {
+        handle.setVisible(visible);
+    }
+
+    @Override
+    public boolean isVisible() {
+        return handle.j;
+    }
+
     @Override
     public void show() {
         handle.setVisible(true);