Do not use guarded cid on externalizable cid-s (one and two byte strings).

runtime/vm/object.h

Index: runtime/vm/object.h
===================================================================
--- runtime/vm/object.h	(revision 25890)
+++ runtime/vm/object.h	(working copy)
@@ -1961,7 +1961,14 @@
   // Return class id that any non-null value read from this field is guaranteed
   // to have or kDynamicCid if such class id is not known.
   // Stores to this field must update this information hence the name.
-  intptr_t guarded_cid() const { return raw_ptr()->guarded_cid_; }
+  intptr_t guarded_cid() const {
+    const intptr_t cid = raw_ptr()->guarded_cid_;
+    if (IsExternalizableCid(cid)) {
+      return kDynamicCid;
+    } else {
+      return cid;
+    }
+  }
   void set_guarded_cid(intptr_t cid) const {
     raw_ptr()->guarded_cid_ = cid;
   }
@@ -1969,6 +1976,10 @@
     return OFFSET_OF(RawField, guarded_cid_);
   }
 
+  static bool IsExternalizableCid(intptr_t cid) {
+    return (cid == kOneByteStringCid) || (cid == kTwoByteStringCid);
+  }
+
   // Returns false if any value read from this field is guaranteed to be
   // not null.
   // Internally we is_nullable_ field contains either kNullCid (nullable) or