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 e40d5451faf672b2134f6be410c0e05b3c3bcaf2..aeb46e98a36efa9b6209e1de12be92997f042623 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -3735,7 +3735,7 @@ class String : public Instance {
           ch_(-1) {
     }
 
-    int32_t Current() {
+    int32_t Current() const {
       ASSERT(index_ >= 0);
       ASSERT(index_ < str_.Length());
       return ch_;
@@ -3743,6 +3743,11 @@ class String : public Instance {
 
     bool Next();
 
+    void Reset() {
+      index_ = -1;
+      ch_ = -1;
+    }
+
    private:
     const String& str_;
     intptr_t index_;