mangle.topLevel = true not working

[fishbar]

topLevel Identifiers can't be renamed by setting up babili-preset with mangle.topLevel = true;

let result = babel.transform('let test = 1;', options);

console.log(result.code); // print `let test=1;`,   expect `let a=1;`

is anybody know how to mangle topLevel Identifiers

[boopathi]

Fixed in #383