src/map/Map.js
function (crs, options) {
    switch(crs) {
      case "EPSG:3857":
      return L.CRS.EPSG3857;
      case "EPSG:3395":
      return L.CRS.EPSG3395;
      case "EPSG:4326":
      return L.CRS.EPSG4326;
      default:
        return this._defineMapCRS(crs, options);
    }
  }
function (crs, options) {
    switch(crs) {
      case "EPSG:3857":
      return L.CRS.EPSG3857;
      case "EPSG:3395":
      return L.CRS.EPSG3395;
      case "EPSG:4326":
      return L.CRS.EPSG4326;
      default:
        return this._defineMapCRS(crs, options);
    }
  }
Use default CRS classes for common codes, fallback to custom for all other codes.