constructor_code.js

function assembleLayer(conf) {
  var constructorr=window, path=conf.provider.split(".");
  
  // get a reference to the constructor, by walking the provider path
  for(var i=0,l=path.length;i<l;i++) { constructorr=constructorr[path[i]]; }

  // following constructor code from: http://stackoverflow.com/a/1608546/887883
  // now invoke it
  function F() {
     return constructorr.apply(this, conf.args);
  }
  F.prototype = constructorr.prototype;
  return new F();
}

map.js

for(var i=0,l=layers.length;i<l;i++) {
  map.addLayer(assembleLayer(layers[i]));
}