code: function getNamespaceConstraints() {
      var map = {};
      getModuleNames(m).forEach(function(n) {
        var split = n.split(':');
        var moduleName = split[0];
        var namespaceName = split[1];
        if (namespaceName) {
          if (SPLIT_MODULES.indexOf(moduleName) === -1) {
            warn('Module ' + moduleName + ' is not ready to be split!');
            warn('Exiting...');
            process.exit();
          }
          var constraints = map[moduleName] || {};
          constraints[namespaceName] = true;
          map[moduleName] = constraints;
        }
      });
      return map;
    }
docs: Allowing namespace constraints such as ES6:String to only build for that namespace.