fix corner case in unused (#5847)

fixes #5843

lib/compress.js

@@ -8284,6 +8284,7 @@ Compressor.prototype.compress = function(node) {
                             if (prop instanceof AST_Spread) return prop;
                             var key = prop_keys[index];
                             if (key instanceof AST_Node) return prop;
+                            if (key === "__proto__") return prop;
                             if (drop_keys.has(key)) {
                                 var mapped = drop_keys.get(key);
                                 if (!mapped) return prop;
@@ -8317,7 +8318,10 @@ Compressor.prototype.compress = function(node) {
                         if (value.has_side_effects(compressor) && prop.value.match_symbol(function(node) {
                             return node instanceof AST_PropAccess;
                         })) break;
-                        value = make_node(AST_Sub, node, {
+                        value = is_identifier_string(prop.key) ? make_node(AST_Dot, node, {
+                            expression: value,
+                            property: prop.key,
+                        }) : make_node(AST_Sub, node, {
                             expression: value,
                             property: make_node_from_constant(prop.key, prop),
                         });