Add asserts to kqueue filter/events on mac.

runtime/bin/eventhandler_macos.cc

Index: runtime/bin/eventhandler_macos.cc
diff --git a/runtime/bin/eventhandler_macos.cc b/runtime/bin/eventhandler_macos.cc
index c884e587557fdf0e6b765b3b0bcc33bed2c108bd..fbced64683ed5af4894d8f780354962c58a50bcb 100644
--- a/runtime/bin/eventhandler_macos.cc
+++ b/runtime/bin/eventhandler_macos.cc
@@ -273,6 +273,8 @@ intptr_t EventHandlerImplementation::GetEvents(struct kevent* event,
         }
       }
       if (event_mask == 0) event_mask |= (1 << kInEvent);
+    } else {
+      UNREACHABLE();
     }
   } else {
     // Prioritize data events over close and error events.
@@ -287,9 +289,7 @@ intptr_t EventHandlerImplementation::GetEvents(struct kevent* event,
         }
         sd->MarkClosedRead();
       }
-    }
-
-    if (event->filter == EVFILT_WRITE) {
+    } else if (event->filter == EVFILT_WRITE) {
       if ((event->flags & EV_EOF) != 0) {
         if (event->fflags != 0) {
           event_mask |= (1 << kErrorEvent);
@@ -304,6 +304,8 @@ intptr_t EventHandlerImplementation::GetEvents(struct kevent* event,
       } else {
         event_mask |= (1 << kOutEvent);
       }
+    } else {
+      UNREACHABLE();
     }
   }