loop variable incorrect when changed in loop block with let scope

buble.surge.sh

Before:

lines = lines.map(function(line) {
  if (line.indexOf(' ') >= 0)
    for (let lineLen = line.length; lineLen < len; )
       line = line.replace(/ +/g, function(spaces) {
         return spaces + (lineLen++ < len ? ' ' : '');
       });
   return line;
 });

After:

lines = lines.map(function(line) {
  if (line.indexOf(' ') >= 0)
    var loop = function ( lineLen ) {
        line = line.replace(/ +/g, function(spaces) {
         return spaces + (lineLen++ < len ? ' ' : '');
       });
      };

      for (var lineLen = line.length; lineLen < len; )
       loop( lineLen );
   return line;
 });