[vm] Support flow events in Dart_TimelineEvent.

runtime/vm/timeline.h

Index: runtime/vm/timeline.h
diff --git a/runtime/vm/timeline.h b/runtime/vm/timeline.h
index 0abf0254d91b0cdca4c8dda9dccb7fbfae28cc9a..34e169f0dd9d0c4d4a5bef2fc98b5ef192f1062c 100644
--- a/runtime/vm/timeline.h
+++ b/runtime/vm/timeline.h
@@ -158,6 +158,9 @@ class TimelineEvent {
     kAsyncInstant,
     kAsyncEnd,
     kCounter,
+    kFlowBegin,
+    kFlowStep,
+    kFlowEnd,
     kMetadata,
     kNumEventTypes,
   };
@@ -210,6 +213,16 @@ class TimelineEvent {
   void Counter(const char* label,
                int64_t micros = OS::GetCurrentMonotonicMicros());
 
+  void FlowBegin(const char* label,
+                 int64_t async_id,
+                 int64_t micros = OS::GetCurrentMonotonicMicros());
+  void FlowStep(const char* label,
+                int64_t async_id,
+                int64_t micros = OS::GetCurrentMonotonicMicros());
+  void FlowEnd(const char* label,
+               int64_t async_id,
+               int64_t micros = OS::GetCurrentMonotonicMicros());
+
   void Metadata(const char* label,
                 int64_t micros = OS::GetCurrentMonotonicMicros());