patch export default within sandbox correctly (#5346)

fixes #5345

test/input/reduce/export_default.js

@@ -0,0 +1,17 @@
+var unused;
+export default class {
+    ____11111() {
+        a, b, c, d, e;
+        f, g, h, i, j;
+        k, l, m, n, o;
+        p, q, r, s, t;
+        u, v, w, x, y, z;
+        A, B, C, D, E;
+        F, G, H, I, J;
+        K, L, M, N, O;
+        P, Q, R, S, T;
+        U, V, W, X, Y, Z;
+        $, _;
+        unused;
+    }
+}

test/mocha/reduce.js

@@ -434,4 +434,18 @@ describe("test/reduce.js", function() {
             "// }",
         ].join("\n"));
     });
+    it("Should transform `export default` correctly", function() {
+        var result = reduce_test(read("test/input/reduce/export_default.js"), {
+            compress: false,
+            toplevel: true,
+        });
+        if (result.error) throw result.error;
+        assert.strictEqual(result.code, [
+            "// Can't reproduce test failure",
+            "// minify options: {",
+            '//   "compress": false,',
+            '//   "toplevel": true',
+            "// }",
+        ].join("\n"));
+    });
 });

test/sandbox.js

@@ -56,7 +56,10 @@ exports.patch_module_statements = function(code) {
     code = code.replace(/\bexport(?:\s*\{[^{}]*}\s*?(?:$|\n|;)|\s+default\b(?:\s*(\(|\{|class\s*\{|class\s+(?=extends\b)|(?:async\s+)?function\s*(?:\*\s*)?\())?|\b)/g, function(match, header) {
         if (!header) return "";
         if (header.length == 1) return "0, " + header;
-        return header.slice(0, -1) + " _" + ++count + header.slice(-1);
+        do {
+            var name = "_export_default_" + ++count;
+        } while (code.indexOf(name) >= 0);
+        return header.slice(0, -1) + " " + name + header.slice(-1);
     }).replace(/\bimport\.meta\b/g, function() {
         return '({ url: "https://example.com/path/index.html" })';
     }).replace(/\bimport\b(?:\s*([^\s('"][^('"]*)\bfrom\b)?\s*(['"]).*?\2(?:$|\n|;)/g, function(match, symbols) {