Added error message for flutter_gallery transitions_perf test. (flutter#47776)

Author: linxuebin1990

examples/flutter_gallery/test_driver/transitions_perf_test.dart

@@ -103,6 +103,8 @@ Future<void> saveDurationsHistogram(List<Map<String, dynamic>> events, String ou
 
   if (unexpectedValueCounts.isNotEmpty) {
     final StringBuffer error = StringBuffer('Some routes recorded wrong number of values (expected 2 values/route):\n\n');
+    // When run with --trace-startup, the VM stores trace events in an endless buffer instead of a ring buffer.
+    error.write('You must add the --trace-startup parameter to run the test. \n\n');
     unexpectedValueCounts.forEach((String routeName, int count) {
       error.writeln(' - $routeName recorded $count values.');
     });