Fix some lint errors.

static/babystats.js

@@ -295,6 +295,7 @@ BabyStats.prototype.setTransitions_ = function() {
  * @private
  * @param {Element} stylesheet
  * @param {string} selector
+ * @return {CSSRule}
  */
 BabyStats.prototype.addStyle_ = function(stylesheet, selector) {
   stylesheet.sheet.insertRule(selector + ' {}', 0);
@@ -575,6 +576,9 @@ BabyStats.prototype.buildLayout_ = function() {
     }.bind(this));
   }.bind(this));
 
+  this.displayTimelines_ = document.createElement('babyStatsDisplayTimelines');
+  back.appendChild(this.displayTimelines_);
+
   var flip = document.createElement('img');
   this.addCSSClass_(flip, 'babyStatsFlip');
   flip.src = '/static/flip.svg';
@@ -704,7 +708,7 @@ BabyStats.prototype.buildGrid_ = function() {
 /**
  * @private
  * @param {number} seconds
- * @param {?function} opt_floatToInt
+ * @param {function(number):number=} opt_floatToInt
  * @return {string}
  */
 BabyStats.prototype.secondsToHuman_ = function(seconds, opt_floatToInt) {