function findKey(object, predicate) {
      return baseFindKey(object, getIteratee(predicate, 3), baseForOwn);
    }
This method is like `_.find` except that it returns the key of the first
element `predicate` returns truthy for instead of the element itself.

@static
@memberOf _
@since 1.1.0
@category Object
@param {Object} object The object to inspect.
@param {Function} [predicate=_.identity] The function invoked per iteration.
@returns {string|undefined} Returns the key of the matched element,
 else `undefined`.
@example

var users = {
  'barney':  { 'age': 36, 'active': true },
  'fred':    { 'age': 40, 'active': false },
  'pebbles': { 'age': 1,  'active': true }
};

_.findKey(users, function(o) { return o.age < 40; });
// => 'barney' (iteration order is not guaranteed)

// The `_.matches` iteratee shorthand.
_.findKey(users, { 'age': 1, 'active': true });
// => 'pebbles'

// The `_.matchesProperty` iteratee shorthand.
_.findKey(users, ['active', false]);
// => 'fred'

// The `_.property` iteratee shorthand.
_.findKey(users, 'active');
// => 'barney'