Make JS util test changes to reflect the annotation updates. (#2447)

* Make JS util test changes to reflect the annotation updates (like not null parameters). This is followup to recent cl/300367316.

* Add non-nullable modifier to return type of functions never returning null. More details in cl/300838092

* Removing blank line.

javascript/i18n/phonenumbers/phonenumberutil.js

@@ -4334,7 +4334,7 @@ i18n.phonenumbers.PhoneNumberUtil.prototype.buildNationalNumberForParsing_ =
  *
  * @param {i18n.phonenumbers.PhoneNumber} numberIn number that we want to copy
  *     fields from.
- * @return {i18n.phonenumbers.PhoneNumber} number with core fields only.
+ * @return {!i18n.phonenumbers.PhoneNumber} number with core fields only.
  * @private
  */
 i18n.phonenumbers.PhoneNumberUtil.copyCoreFieldsOnly_ = function(numberIn) {

javascript/i18n/phonenumbers/shortnumberinfo.js

@@ -385,7 +385,7 @@ i18n.phonenumbers.ShortNumberInfo.prototype.getRegionCodeForShortNumberFromRegio
 
 /**
  * Convenience method to get a list of what regions the library has metadata for
- * @return {Array<string>} the list of region codes
+ * @return {!Array<string>} the list of region codes
  * @package
  */
 i18n.phonenumbers.ShortNumberInfo.prototype.getSupportedRegions = function() {