Add support for surrogates when serializing and deserializing for native ports

runtime/vm/object.h

Index: runtime/vm/object.h
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index 6a62d7f31a9514b57d085b793e996fab1f2555f3..2b43f970ddbf794d8eccee97cd33a6ddcfac9262 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -3725,7 +3725,7 @@ class String : public Instance {
       ASSERT(end_ <= str.Length());
     }
 
-    int32_t Current() {
+    int32_t Current() const {
       ASSERT(index_ >= 0);
       ASSERT(index_ < end_);
       return ch_;