use threadpool on C API

ios/include/dart/internal/dart_api_dl_impl.h

@@ -0,0 +1,21 @@
+/*
+ * Copyright (c) 2020, 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.
+ */
+
+#ifndef RUNTIME_INCLUDE_INTERNAL_DART_API_DL_IMPL_H_
+#define RUNTIME_INCLUDE_INTERNAL_DART_API_DL_IMPL_H_
+
+typedef struct {
+  const char* name;
+  void (*function)(void);
+} DartApiEntry;
+
+typedef struct {
+  const int major;
+  const int minor;
+  const DartApiEntry* const functions;
+} DartApi;
+
+#endif /* RUNTIME_INCLUDE_INTERNAL_DART_API_DL_IMPL_H_ */ /* NOLINT */