[ast] Don't assume declaration scopes are never removed.

src/ast/scopes.cc

Index: src/ast/scopes.cc
diff --git a/src/ast/scopes.cc b/src/ast/scopes.cc
index a41d34303291da4fddd418c4d6a67ca5e8569310..05b09cd86a17d108390bbef11a7df648d78a52bc 100644
--- a/src/ast/scopes.cc
+++ b/src/ast/scopes.cc
@@ -638,7 +638,7 @@ Variable* DeclarationScope::DeclareFunctionVar(const AstRawString* name) {
 bool Scope::HasBeenRemoved() const {
   // TODO(neis): Store this information somewhere instead of calculating it.
 
-  if (!is_block_scope() || is_declaration_scope()) return false;
+  if (!is_block_scope()) return false;  // Shortcut.
 
   Scope* parent = outer_scope();
   if (parent == nullptr) {