[vm] Add a stub Dart_NotifyIdle() for the embedder to suggest a good time to do GC.

runtime/include/dart_api.h

Index: runtime/include/dart_api.h
diff --git a/runtime/include/dart_api.h b/runtime/include/dart_api.h
index e54a52d519adc1bbee5bd6edf539f23ff420e9b9..57d19192401e226ffafe788e670222d335bf1bf9 100644
--- a/runtime/include/dart_api.h
+++ b/runtime/include/dart_api.h
@@ -954,6 +954,18 @@ DART_EXPORT Dart_Handle Dart_DebugName();
  */
 DART_EXPORT void Dart_EnterIsolate(Dart_Isolate isolate);
 
+/**
+ * Notifies the VM that the embedder expects to be idle until |deadline|. The VM
+ * may use this time to perform garbage collection or other tasks to avoid
+ * delays during execution of Dart code in the future.
+ *
+ * |deadline| is measured in microseconds against the system's monotonic time.
+ * This clock can be accessed via Dart_TimelineGetMicros().
+ *
+ * Requires there to be a current isolate.
+ */
+DART_EXPORT void Dart_NotifyIdle(int64_t deadline);
+
 /**
  * Notifies the VM that the current thread should not be profiled until a
  * matching call to Dart_ThreadEnableProfiling is made.