code: function(root, transition, applicationHost) {
              var hostElement, settings = { activate:true, transition: transition };
  
              if (!applicationHost || system.isString(applicationHost)) {
                  hostElement = document.getElementById(applicationHost || 'applicationHost');
              } else {
                  hostElement = applicationHost;
              }
  
              if (system.isString(root)) {
                  if (viewEngine.isViewUrl(root)) {
                      settings.view = root;
                  } else {
                      settings.model = root;
                  }
              } else {
                  settings.model = root;
              }
  
              composition.compose(hostElement, settings);
          }
docs: Sets the root module/view for the application.
  @method setRoot
  @param {string} root The root view or module.
  @param {string} [transition] The transition to use from the previous root (or splash screen) into the new root.
  @param {string} [applicationHost] The application host element or id. By default the id 'applicationHost' will be used.