fix corner case in evaluate (#5729)

fixes #5728

lib/output.js

@@ -1783,6 +1783,9 @@ function OutputStream(options) {
         output.print(self.strings[i]);
         output.print("`");
     });
+    DEFPRINT(AST_BigInt, function(output) {
+        output.print(this.value + "n");
+    });
     DEFPRINT(AST_Constant, function(output) {
         output.print("" + this.value);
     });

lib/parse.js

@@ -391,7 +391,8 @@ function tokenizer($TEXT, filename, html5_comments, shebang) {
         var valid = parse_js_number(num);
         if (isNaN(valid)) parse_error("Invalid syntax: " + num);
         if (has_dot || has_e || peek() != "n") return token("num", valid);
-        return token("bigint", num.toLowerCase() + next());
+        next();
+        return token("bigint", num.toLowerCase());
     }
 
     function read_escaped_char(in_string) {

test/compress/bigint.js

@@ -90,3 +90,17 @@ issue_4801: {
     expect_stdout: "PASS"
     node_version: ">=10.4.0"
 }
+
+issue_5728: {
+    options = {
+        evaluate: true,
+    }
+    input: {
+        console.log("" + 4n + 2);
+    }
+    expect: {
+        console.log("42");
+    }
+    expect_stdout: "42"
+    node_version: ">=10.4.0"
+}