[GHC] #4123: Control.Concurrent.MVar doesn't need to depend on the Prelude

[GHC]

#4123: Control.Concurrent.MVar doesn't need to depend on the Prelude
---------------------------------+------------------------------------------
    Reporter:  tibbe             |       Owner:                     
        Type:  bug               |      Status:  new                
    Priority:  normal            |   Component:  libraries/base     
     Version:  6.13              |    Keywords:                     
          Os:  Unknown/Multiple  |    Testcase:                     
Architecture:  Unknown/Multiple  |     Failure:  Building GHC failed
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 `Control.Concurrent.MVar` currently depends on the `Prelude`, causing an
 import cycle in the new I/O manager. This patch replaces the dependency on
 the `Prelude` with modules in the `GHC` hierarchy, breaking the cycle.

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4123>
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