Fix crashing bug due to the fact that external one byte strings cannot be escaped

runtime/vm/object.h

Index: runtime/vm/object.h
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index a1272a711c91598a24e1fdb0c6143e5b143a592e..a5a740108070f6d54b39b609d92e76d2d6b2b8dc 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -4943,6 +4943,7 @@ class OneByteString : public AllStatic {
 
   friend class Class;
   friend class String;
+  friend class ExternalOneByteString;
   friend class SnapshotReader;
 };
 
@@ -5064,6 +5065,8 @@ class ExternalOneByteString : public AllStatic {
     return reinterpret_cast<RawExternalOneByteString*>(Object::null());
   }
 
+  static RawOneByteString* EscapeSpecialCharacters(const String& str);
+
   static const ClassId kClassId = kExternalOneByteStringCid;
 
  private: