SQL Server 2012 :: Run DBCC Inside Stored Proc Using EXECUTE AS

I am a SysAdmin on a test/dev SQL instance. Other non-sysadmin users (developers) need the ability to execute DBCC commands like the following:

DBCC FREESYSTEMCACHE ('ALL') WITH MARK_IN_USE_FOR_REMOVAL
OR
DBCC FREEPROCCACHE

I tried creating a store proc in a user database and granting those non-sysadmin users EXECUTE permission on it as so:

CREATE PROC spFreeSystemCache
WITH EXECUTE AS 'sa'
AS
DBCC FREESYSTEMCACHE ('ALL') WITH MARK_IN_USE_FOR_REMOVAL
GO

When I try to create this proc, I get the following error:

Msg 102, Level 15, State 1, Procedure spFreeSystemCache, Line 2
Incorrect syntax near 'sa'.

Ok, so I can't EXECUTE AS sa...