Add trace_service_pause_events flag and related tracing

runtime/bin/vmservice/server.dart

Index: runtime/bin/vmservice/server.dart
diff --git a/runtime/bin/vmservice/server.dart b/runtime/bin/vmservice/server.dart
index d622e62dbeb6c164585825209712f4b46faf9122..051ea43ff64eed6d97a83ea3b0b961a65516dae4 100644
--- a/runtime/bin/vmservice/server.dart
+++ b/runtime/bin/vmservice/server.dart
@@ -127,7 +127,7 @@ class Server {
       _server = s;
       _server.listen(_requestHandler);
       if (display_message) {
-        print('VMService listening on $ip:$port');
+        print('Observatory listening on http://$ip:$port');
       }
       return s;
     });