Using es6 classes mixins, some functions are lost

[pinguxx]

1. Describe your issue:
   I have a es6 class that extend the riot.Tag with a mixin that is also a es6 class, the tag has a template with an onclick, but the function dissapears after i do this.mixin oncreate

i notice that on the line 2113 in riot.js the if block for the keys, its letting pass proto which is destroying the prototype of the class, maybe you can add if key !== 'init' && key !== '__proto__'

Maybe im doing something wrong, or the this.mixin should be in some other place

2. Can you reproduce the issue?
   http://plnkr.co/edit/XnlTilvmoepk7egeiMDQ?p=preview

3. On which browser/OS does the issue appear?
   Chrome, ff, ie

4. Which version of Riot does it affect?
   3.0.7

5. How would you tag this issue?

• Question
• Bug
• Discussion
• Feature request
• Tip
• Enhancement
• Performance