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

Because they sometimes are.

R=rossberg@chromium.org
BUG=v8:5648

Review-Url: https://codereview.chromium.org/2524333002
Cr-Commit-Position: refs/heads/master@{#41262}

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) {