transition mixin -> flex attribute -> no prefixes

[nickwinger]

Hi,

we are using CSS3 transitions on the flex attribute. To support all browsers the flex attribute inside the transition needs to be prefixed.
With the .transition mixin this is not working, as only the normal flex attribute gets generated but not the ones with the prefixes. For example the filter attribute inside the .transition mixin is working with all prefixes.

Thanks,
Cheers,
Nick