Scope analysis: DCHECK that we allocate rest_parameter_ only once.

src/ast/scopes.cc

Index: src/ast/scopes.cc
diff --git a/src/ast/scopes.cc b/src/ast/scopes.cc
index 1330f79b28d4d9373fbba2df76a9a95eea04424e..09474357b9fd84852e5ab026d6d96fecbd31e605 100644
--- a/src/ast/scopes.cc
+++ b/src/ast/scopes.cc
@@ -1678,9 +1678,7 @@ void DeclarationScope::AllocateLocals() {
     AllocateNonParameterLocal(function_);
   }
 
-  if (rest_parameter_ != nullptr) {
-    AllocateNonParameterLocal(rest_parameter_);
-  }
+  DCHECK(rest_parameter_ == nullptr || !rest_parameter_->IsUnallocated());
 
   if (new_target_ != nullptr && !MustAllocate(new_target_)) {
     new_target_ = nullptr;