Issue: button disappear in "ion-footer-bar"

[Paul-Lo]

Issue Repro steps:

1. open activity.html of Thronester Demo
2. add the following <ion-footer-bar> between </ion-content> and </ion-view>
<ion-footer-bar class="bar-light">
<button class="button button-positive button-clear no-animation" >
Click Me
</button>
</ion-footer-bar>
3. run the demo project and go to Activity, you'll find that the button in ion-footer-bar disappears.
4. but if you put the same button within <ion-content></ion-content> of the activity.html, it shows up normally.
   <button class="button button-positive button-clear no-animation">
Click Me
</button>

Why the same button disappear in <ion-footer-bar>?

Thanks.