Error messages on dashboard page in Bartik theme

Steps to reproduce:

1. Set your admin theme to bartik one way or another, either by explicitly setting it or setting it to your 'default' theme which you have set to bartik.

2. Go to the dashboard page, assign a few blocks here and there.

Now, each time you view the dashboard page, you get a flood of error messages (Notice: Undefined property: stdClass::$position_first in bartik_preprocess_block() on line 120) (Notice: Notice: Undefined property: stdClass::$position_last in bartik_preprocess_block() on line 123) (Notice: Undefined property: stdClass::$position in bartik_preprocess_block() on line 120).

Looked through the issue queue a bit but couldn't seem to find another issue for this; I do apologize if this is a duplicate. I marked this as a problem with dashboard.module, though this could just as easily be a problem with Bartik.

This is a pretty big one because we can't have the main administration page throwing tons of errors if you view it in our default theme.