[builtins] don't pre-flatten strings for JSStringIterator

src/objects-debug.cc

Index: src/objects-debug.cc
diff --git a/src/objects-debug.cc b/src/objects-debug.cc
index 5902e0dda8cd6cc0ac47362849e6aa9a381f7a26..3c43f230745e539046e24005c12bfa6ee05c725b 100644
--- a/src/objects-debug.cc
+++ b/src/objects-debug.cc
@@ -785,7 +785,7 @@ void JSWeakMap::JSWeakMapVerify() {
 void JSStringIterator::JSStringIteratorVerify() {
   CHECK(IsJSStringIterator());
   JSObjectVerify();
-  CHECK(string()->IsSeqString() || string()->IsExternalString());
+  CHECK(string()->IsString());
 
   CHECK_GE(index(), 0);
   CHECK_LE(index(), String::kMaxLength);