According to MSDN, the ListBox class should be implemented in PresentationFramework.dll. However, I cannot find this DLL in the WP8 SDK emulator images.
What DLL contains the WP ListBox control?
UPDATE: After asking this question, I did some research and found the ListBox implementation in System.Windows.dll (System.Windows.ni.dll). But now the question turns into the following: why it does not correspond to the MSDN documentation and what online source should I use to know the real location of a class in Silverlight for WP?
You should still use MSDN, but I stumbled upon a few problems such as yours with it. The reason for this is because some classes come from a different dll wether you use it in Windows 8 developpment or windows phone 8 developpment
In this case, the msdn link for ListBox in WP8 is here
There is no real way to find your answer everytime (at least I didn't find one) but when the classic library doesnt work, I google the name of the library + windows phone 8 and it usually helps me finding the good one. Here it would be "ListBox windows phone 8", that's how I found the MSDN link
Hope it helps