Steps to reproduce and a minimal demo of the problem

http://plnkr.co/edit/Sb5d625JwLg9Yq8J6Uzh?p=preview

Current behavior

Only the border class is added when evaluating the following expression: <div [class]="'red' + (true ? ' border' : '')">

Expected/desired behavior

We should see a red rectangle with a border - it should have both red and border classes.

Other information

This is a regression from beta.15 where it used to work without problems.