fix unused crashes (#1599)

- `AST_DefaultAssign` on `keep_fargs`
- `AST_Expansion on` `keep_fargs`
- `AST_Destructuring` on top-level declarations without `toplevel`

lib/ast.js

@@ -814,9 +814,6 @@ var AST_VarDef = DEFNODE("VarDef", "name value", {
         name: "[AST_SymbolVar|AST_SymbolConst|AST_Destructuring] name of the variable",
         value: "[AST_Node?] initializer, or null of there's no initializer"
     },
-    is_destructuring: function() {
-        return this.name instanceof AST_Destructuring;
-    },
     _walk: function(visitor) {
         return visitor._visit(this, function(){
             this.name._walk(visitor);