Quick fix: nullify Isolate in background parsing slightly later.

src/parsing/parser-base.h

Index: src/parsing/parser-base.h
diff --git a/src/parsing/parser-base.h b/src/parsing/parser-base.h
index 000b606a9549570b50ca9429633a5d88e6dbbf3f..7ef10ebd270d137ca3d4bcc5e811d13b7b7e303e 100644
--- a/src/parsing/parser-base.h
+++ b/src/parsing/parser-base.h
@@ -252,6 +252,8 @@ class ParserBase : public Traits {
 
   uintptr_t stack_limit() const { return stack_limit_; }
 
+  void set_stack_limit(uintptr_t stack_limit) { stack_limit_ = stack_limit; }
+
  protected:
   enum AllowRestrictedIdentifiers {
     kAllowRestrictedIdentifiers,