This extra stage run immediately after AST creation and process following tasks: - ~~Desugaring for top-level non-exported to host, non-mutable (const) `FunctionExpression` to `FunctionDeclaration` with preserving order.~~ - Destructive assignment desugaring - Spread syntax desugaring - Variadic function desugaring - String enum desugaring (?) - Processing for custom (user-space) decorators