[vm] Support flow events in Dart_TimelineEvent.

runtime/include/dart_tools_api.h

Index: runtime/include/dart_tools_api.h
diff --git a/runtime/include/dart_tools_api.h b/runtime/include/dart_tools_api.h
index 270ce8140f90f0008223c5134f534f6845419fa8..29f86a9a2c5fa49232387f0f8af182ede6589b77 100644
--- a/runtime/include/dart_tools_api.h
+++ b/runtime/include/dart_tools_api.h
@@ -1024,6 +1024,9 @@ typedef enum {
   Dart_Timeline_Event_Async_End,      // Phase = 'e'.
   Dart_Timeline_Event_Async_Instant,  // Phase = 'n'.
   Dart_Timeline_Event_Counter,        // Phase = 'C'.
+  Dart_Timeline_Event_Flow_Begin,     // Phase = 's'.
+  Dart_Timeline_Event_Flow_Step,      // Phase = 't'.
+  Dart_Timeline_Event_Flow_End,       // Phase = 'f'.
 } Dart_Timeline_Event_Type;
 
 /**