Use isset() instead of array_key_exists() in DIC

It doesn't look like the array_key_exists() in Container::get() and Container::has() is necessary. Changing this to isset() removed over 1,000 calls on a default Drupal 8 install - attaching before/after xhprof screenshots with this change.

![screen shot 2013-09-01 at 1 13 41 pm](https://f.cloud.github.com/assets/116285/1063965/d826057c-1300-11e3-96c3-2c94b89fe83a.png)

![screen shot 2013-09-01 at 1 13 49 pm](https://f.cloud.github.com/assets/116285/1063961/99a0f7b2-1300-11e3-9bd2-b0bf22d4245e.png)