cannot handle default arguments for lamdba
this code does't work.
const buble = require("buble");
const source = `
(function(x, f = () => x) {
var x;
var y = x;
x = 2;
return [x, y, f()];
})(1)
`;
var result = buble.transform( source );
console.log(result.code);
eval(result.code);