Debugger support for step-into async and async* functions.

runtime/lib/async.cc

+// Copyright (c) 2017, 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 "vm/bootstrap_natives.h"
+#include "vm/debugger.h"
+#include "vm/exceptions.h"
+#include "vm/native_entry.h"
+#include "vm/object_store.h"
+#include "vm/runtime_entry.h"
+
+namespace dart {
+
+DEFINE_NATIVE_ENTRY(AsyncStarMoveNext_debuggerStepCheck, 1) {
+  GET_NON_NULL_NATIVE_ARGUMENT(Closure, async_op, arguments->NativeArgAt(0));
+  if (FLAG_support_debugger) {
+    Debugger* debugger = isolate->debugger();
+    if (debugger != NULL) {
+      debugger->MaybeAsyncStepInto(async_op);
+    }
+  }
+  return Object::null();
+}
+
+}  // namespace dart