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

runtime/vm/dart_api_impl.cc

Index: runtime/vm/dart_api_impl.cc
diff --git a/runtime/vm/dart_api_impl.cc b/runtime/vm/dart_api_impl.cc
index 4bc1125f23058b732b486cee122f850f03dd89db..52e892cfd7248294d682af1d90ea9d9387cbc2c8 100644
--- a/runtime/vm/dart_api_impl.cc
+++ b/runtime/vm/dart_api_impl.cc
@@ -1453,6 +1453,12 @@ DART_EXPORT Dart_Handle Dart_GetStickyError() {
   return Dart_Null();
 }
 
+DART_EXPORT void Dart_NotifyIdle(int64_t deadline) {
+  Thread* T = Thread::Current();
+  CHECK_ISOLATE(T->isolate());
+  API_TIMELINE_BEGIN_END;
+}
+
 DART_EXPORT void Dart_ExitIsolate() {
   Thread* T = Thread::Current();
   CHECK_ISOLATE(T->isolate());