Explicitly handle listen sockets and connection sockets differently in Linux and Mac OS eventhandler

runtime/bin/eventhandler.h

Index: runtime/bin/eventhandler.h
diff --git a/runtime/bin/eventhandler.h b/runtime/bin/eventhandler.h
index 381ad8ff673c3bb61346e98cec4eaeacad16e9c0..71df2044c4976c947f0e75d8208bebb319eb107f 100644
--- a/runtime/bin/eventhandler.h
+++ b/runtime/bin/eventhandler.h
@@ -18,15 +18,17 @@
 #error Unknown target os.
 #endif
 
-/*
- * Keep these constant in sync with the dart poll event identifiers.
- */
-enum Message {
+// Flags used to provide information and actions to the eventhandler
+// when sending a message about a file descriptor. These flags should
+// be kept in sync with the constants in socket_impl.dart. For more
+// information see the comments in socket_impl.dart
+enum MessageFlags {
   kInEvent = 0,
-  kOutEvent,
-  kErrorEvent,
-  kCloseEvent,
-  kCloseCommand,
+  kOutEvent = 1,
+  kErrorEvent = 2,
+  kCloseEvent = 3,
+  kCloseCommand = 8,
+  kListeningSocket = 16,
 };