Throw event when EnderCrystals harm EnderDragon. Fixes BUKKIT-4052

The EntityLiving dealDamage method will not call an event for the
entity damage caused by an explosion without an associated
entity cause, therefore, the explosion caused by EnderCrystals
needs to be explicitely handled within the EnderDragon class.

src/main/java/net/minecraft/server/EntityEnderDragon.java

@@ -7,6 +7,7 @@ import java.util.List;
 import org.bukkit.block.BlockState;
 import org.bukkit.craftbukkit.util.BlockStateListPopulator;
 import org.bukkit.event.entity.EntityCreatePortalEvent;
+import org.bukkit.event.entity.EntityDamageEvent;
 import org.bukkit.event.entity.EntityExplodeEvent;
 import org.bukkit.event.entity.EntityRegainHealthEvent;
 import org.bukkit.Bukkit;
@@ -300,7 +301,15 @@ public class EntityEnderDragon extends EntityInsentient implements IComplex, IMo
         if (this.bC != null) {
             if (this.bC.dead) {
                 if (!this.world.isStatic) {
-                    this.a(this.bq, DamageSource.explosion((Explosion) null), 10.0F);
+                    // CraftBukkit start
+                    EntityDamageEvent event = new EntityDamageEvent(this.getBukkitEntity(), org.bukkit.event.entity.EntityDamageEvent.DamageCause.ENTITY_EXPLOSION, 10.0F);
+                    Bukkit.getPluginManager().callEvent(event);
+
+                    if (!event.isCancelled()) {
+                        getBukkitEntity().setLastDamageCause(event);
+                        this.a(this.bq, DamageSource.explosion((Explosion) null), (float) event.getDamage());
+                    }
+                    // CraftBukkit end
                 }
 
                 this.bC = null;