test(travis): Disable dart2js/dev tests due to a bug in dart2js

see:
- bug: https://code.google.com/p/dart/issues/detail?id=13523
- failure: https://travis-ci.org/angular/angular.dart/jobs/24455215

This bug is quite old but as explained by @chirayuk in the bug report comments but [this commit](https://github.com/chirayuk/angular.dart/commit/68db30b34a873952d914e73b8a58e48857d6cc8e#diff-9df6a774fe4890fa9b478821b35688fdR1294) triggers it.

_(This should be reverted when the bug gets fixed)_