Fix library private key stability during reload

runtime/vm/isolate_reload.h

Index: runtime/vm/isolate_reload.h
diff --git a/runtime/vm/isolate_reload.h b/runtime/vm/isolate_reload.h
index d439414749f362ba4475ab1397efdb322b1f147a..de6ff001d7f72fbd2ee50d8c48702a3dc60b85f3 100644
--- a/runtime/vm/isolate_reload.h
+++ b/runtime/vm/isolate_reload.h
@@ -68,6 +68,10 @@ class IsolateReloadContext {
 
   void RegisterClass(const Class& new_cls);
 
+  // Finds the library private key for |replacement_or_new| or return null
+  // if |replacement_or_new| is new.
+  RawString* FindLibraryPrivateKey(const Library& replacement_or_new);
+
   int64_t start_time_micros() const { return start_time_micros_; }
 
  private: