Implement safepointing of threads

runtime/vm/native_arguments.h

Index: runtime/vm/native_arguments.h
diff --git a/runtime/vm/native_arguments.h b/runtime/vm/native_arguments.h
index 0655a6914652fab8d29f296ebf7e9b3b152f7327..028b58b5f34165af6fd4ee58653077fa0251a11c 100644
--- a/runtime/vm/native_arguments.h
+++ b/runtime/vm/native_arguments.h
@@ -96,6 +96,10 @@ class NativeArguments {
     return *arg_ptr;
   }
 
+  bool IsNativeAutoSetupScope() const {
+    return AutoSetupScopeBits::decode(argc_tag_);
+  }
+
   int NativeArgCount() const {
     int function_bits = FunctionBits::decode(argc_tag_);
     return ArgCount() - NumHiddenArgs(function_bits);