Added unit tests for the event abstractions.
Added Element as the notion of common ancestry between Device and Host.
diff --git a/net/api/src/test/java/org/onlab/onos/event/BrokenListener.java b/net/api/src/test/java/org/onlab/onos/event/BrokenListener.java
new file mode 100644
index 0000000..dc83419
--- /dev/null
+++ b/net/api/src/test/java/org/onlab/onos/event/BrokenListener.java
@@ -0,0 +1,13 @@
+package org.onlab.onos.event;
+
+/**
+ * Test event listener fixture.
+ */
+public class BrokenListener extends TestListener {
+
+ public void event(TestEvent event) {
+ throw new IllegalStateException("boom");
+ }
+
+}
+