I am able to see the heart icon in a v2.0-alpha.45 app using this syntax. If you are trying to use a v1.0 app please use the syntax here:
<i class="icon ion-heart"></i>
<i class="icon ion-ios-heart"></i>http://ionicframework.com/docs/components/#icons
You can check your version by running ionic info from within the project folder. If this is a v2 error, let me know.