Tests for GuardField length check along with bug fixes

runtime/vm/object.cc

Index: runtime/vm/object.cc
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index f928bcc9866aa0ae75a2b94ea7210386c9c53fc2..1931b9b5016d3ba3b1a857b67c55d1e395d9c09a 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -5355,7 +5355,11 @@ RawField* Field::New(const String& name,
   result.set_has_initializer(false);
   result.set_guarded_cid(kIllegalCid);
   result.set_is_nullable(false);
-  result.set_guarded_list_length(Field::kUnknownFixedLength);
+  if (is_final) {
+    result.set_guarded_list_length(Field::kUnknownFixedLength);
+  } else {
+    result.set_guarded_list_length(Field::kNoFixedLength);
+  }
   result.set_dependent_code(Object::null_array());
   return result.raw();
 }
@@ -5575,7 +5579,6 @@ void Field::UpdateLength(intptr_t list_length) const {
     // List length unchanged.
     return;
   }
-
   // Multiple list lengths assigned here, stop tracking length.
   set_guarded_list_length(Field::kNoFixedLength);
   DeoptimizeDependentCode();