I am running into the same issue. I have just updated to ionic 2 (2.0.0-beta.15) and did a ionic start testApp --v2 to create the default tabs app.
After that I did an ionic serve --lab. I can see the first tab of the app with content but the other tow tabs are empty. i see the same "Uncaught ReferenceError: ionic is not defined". I do have bundle.js included and the browser says it loaded the file successfully.
The reference error occurs in the script tag code, that got injected by ionic into the index.html file:
...
<body>
<script src="cordova.js"></script>
<script src="build/js/app.bundle.js"></script>
<ion-app></ion-app>
<script>
ionic && ionic.Platform && ionic.Platform.setPlatform("android");
</script>
</body>
...Any ideas?