code: function addTriggers(keys, type, f) {
    return (keys && keys.length) ? `Ember.${type}(${dependedKeys(keys)}, ${f})` : f;
  }
docs: Wrap function body (<code>f</code>) into the 'keys'
  Example:
  <pre>
  let keys = ['a', 'b', 'c'];
  let type = 'observes';
  let f = 'function () {}';
  let output = addTriggers(keys, type, f); // "Ember.observes('a', 'b', 'c', function () {})"
  </pre>
  If <code>keys</code> is empty array, <code>f</code> is returned without any changes
  @param {string[]} keys list of the depended keys
  @param {string} type type of dependency ('observes', 'on')
  @param {string} f function's body
  @returns {string}