In Turkish, "i" uppercases to "İ" (note the dot) - there's a separate undotted lowercase "ı" which becomes "I":

http://www.i18nguy.com/unicode/turkish-i18n.html

So basically if your goal is for "i".upperCase() to produce something which goes to another program (e.g. an API call), the Angular code is correct. If it's going to be displayed to humans, it's wrong. The documentation at http://docs.angularjs.org/api/angular.uppercase should probably specify that this cannot be used for user content.