What does だって do in 「それは僕だって知ってる…」?


This is from 楽しいムーミン一家. You can watch the full dialogue
[here](http://www.dailymotion.com/video/x11votv_%E6%A5%BD%E3%81%97%E3%81%84%E3%83%A0%E3%83%BC%E3%83%9F%E3%83%B3%E4%B8%80%E5%AE%B6-34-%E3%81%9F%E3%81%93%E3%81%AB%E4%B9%97%E3%81%A3%E3%81%9F%E3%83%9F%E3%82%A4_shortfilms)
starting from 2:35 if you need more context. The part I'm asking about begins
around 3:25.

Basically, winter is coming so Snufkin is planning to go on a journey as
usual. Moomin doesn't want to part with his friend so he asks Moomin Papa if
he can go together with Snufkin. Papa says:

> スナフキンは孤独を愛している。だから、彼には一人旅がどうしても必要なんだ。
>
> Snufkin loves to be alone. For him it needs to be a solitary journey no
> matter what.

To which Moomin replies:

> それは僕 **だって** 知ってる…

The overall message is undoubtedly "I know that" but what does だって add here?
My guess is that it means "I know that better than anyone else" but I'm not
sure... It doesn't seem to match the dictionary definitions.




This [link](http://ejje.weblio.jp/content/%E3%81%A0%E3%81%A3%E3%81%A6)
definition #1, and this
[link](http://ejje.weblio.jp/sentence/content/%22%E7%A7%81%E3%81%A0%E3%81%A3%E3%81%A6%22).

Translation: " ** _Even_** I know that." or "I **_too_** know that".




This [link](http://ejje.weblio.jp/content/%E3%81%A0%E3%81%A3%E3%81%A6)
definition #1, and this
[link](http://ejje.weblio.jp/sentence/content/%22%E7%A7%81%E3%81%A0%E3%81%A3%E3%81%A6%22).

Translation: " ** _Even_** I know that." or "I **_too_** know that".

