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

runtime/vm/dart_api_impl.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "include/dart_api.h"
 #include "include/dart_mirrors_api.h"
 #include "include/dart_native_api.h"
 
 #include "lib/stacktrace.h"
 #include "platform/assert.h"
@@ skipping 1435 matching lines @@
   Isolate* I = T->isolate();
   CHECK_ISOLATE(I);
   NoSafepointScope no_safepoint_scope;
   if (I->sticky_error() != Error::null()) {
     Dart_Handle error = Api::NewHandle(T, I->sticky_error());
     return error;
   }
   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());
   // The Thread structure is disassociated from the isolate, we do the
   // safepoint transition explicitly here instead of using the TransitionXXX
   // scope objects as the original transition happened outside this scope in
   // Dart_EnterIsolate/Dart_CreateIsolate.
   ASSERT(T->execution_state() == Thread::kThreadInNative);
   T->ExitSafepoint();
   T->set_execution_state(Thread::kThreadInVM);
@@ skipping 5304 matching lines @@
 #endif
 }
 
 DART_EXPORT void Dart_DumpNativeStackTrace(void* context) {
 #ifndef PRODUCT
   Profiler::DumpStackTrace(context);
 #endif
 }
 
 }  // namespace dart