Use a common subroutine for the snapshot reading and writing of strings.

runtime/vm/object.h

Index: runtime/vm/object.h
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index d5e8434356f4ab5049afcb8106f22f4c46fd7f69..62e715efc365617db90982c60b31ff8c2357ae5c 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -2489,6 +2489,11 @@ class String : public Instance {
     raw_ptr()->hash_ = Smi::New(value);
   }
 
+  template<typename HandleType, typename ElementType>
+  static RawString* ReadFromImpl(SnapshotReader* reader,
+                                 intptr_t object_id,
+                                 bool classes_serialized);
+
   HEAP_OBJECT_IMPLEMENTATION(String, Instance);
 };