src/polarmap.js
function(projection) {
  for (var key in tiles) {
    if (tiles.hasOwnProperty(key)) {
      if (key.indexOf(projection) !== -1) {
        return tiles[key];
      }
    }
  }

  return null;
}
function(projection) {
  for (var key in tiles) {
    if (tiles.hasOwnProperty(key)) {
      if (key.indexOf(projection) !== -1) {
        return tiles[key];
      }
    }
  }

  return null;
}
Return the tile layer that has a key containing `projection`