patch export default within sandbox correctly (#5400)

fixes #5399

test/mocha/reduce.js

@@ -434,7 +434,7 @@ describe("test/reduce.js", function() {
             "// }",
         ].join("\n"));
     });
-    it("Should transform `export default` correctly", function() {
+    it("Should transform `export default class` correctly", function() {
         var result = reduce_test(read("test/input/reduce/export_default.js"), {
             compress: false,
             toplevel: true,
@@ -448,4 +448,22 @@ describe("test/reduce.js", function() {
             "// }",
         ].join("\n"));
     });
+    it("Should transform `export default function` correctly", function() {
+        var code = [
+            "for (var k in this)",
+            "    console.log(k);",
+            "export default (function f() {});",
+            "console.log(k);",
+        ].join("\n");
+        var result = reduce_test(code, {
+            mangle: false,
+        });
+        if (result.error) throw result.error;
+        assert.strictEqual(result.code, [
+            "// Can't reproduce test failure",
+            "// minify options: {",
+            '//   "mangle": false',
+            "// }",
+        ].join("\n"));
+    });
 });