Description:
------------
We have a setup where we have multiple client base model 
classes which are aliased to a common name with class_alias 
depending on which client the current request pertains to.  
These "virtual" classes are then extended by the core 
application model classes.

When APC is enabled the application classes always extend the 
client model which was aliased at the time that include was 
cached.

I have also reproduced with 3.1.7.

Please see the gist below for a simple example.

Reproduce code:
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https://gist.github.com/789541

Expected result:
----------------
Repeated runs should expect and get a model for the same 
client:

Expecting model for client a
Got model for client a

Expecting model for client b
Got model for client b

Actual result:
--------------
Repeated runs always get a model for the client chosen in the 
first run:

Expecting model for client a
Got model for client a

Expecting model for client b
Got model for client a