move code to src/test

* so that it compiles and tests pass
* fix some additional snip references in getting started

akka-docs/src/test/java/jdocs/io/japi/Watcher.java

@@ -0,0 +1,37 @@
+package jdocs.io.japi;
+
+import java.util.concurrent.CountDownLatch;
+
+import akka.actor.ActorRef;
+import akka.actor.Terminated;
+import akka.actor.AbstractActor;
+
+public class Watcher extends AbstractActor {
+  
+  static public class Watch {
+    final ActorRef target;
+    public Watch(ActorRef target) {
+      this.target = target;
+    }
+  }
+  
+  final CountDownLatch latch;
+
+  public Watcher(CountDownLatch latch) {
+    this.latch = latch;
+  }
+
+  @Override
+  public Receive createReceive() {
+    return receiveBuilder()
+      .match(Watch.class, msg -> {
+        getContext().watch(msg.target);
+      })
+      .match(Terminated.class, msg -> {
+        latch.countDown();
+        if (latch.getCount() == 0) getContext().stop(getSelf());
+      })
+      .build();
+  }
+
+}