function validate_(params) {
  Object.keys(params).forEach(function(name) {
    var value = params[name];
    if (!value) {
      throw new Error(name + ' is required.');
    }
  });
}
Validates that all of the values in the object are non-empty. If an empty
value is found, and error is thrown using the key as the name.
@param {Object.<string, string>} params The values to validate.
@private