[GHC] #11214: Remove JavaScriptFFI from --supported-extensions list

[GHC]

#11214: Remove JavaScriptFFI from --supported-extensions list
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           Reporter:  luite          |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 GHC supports the `JavaScriptFFI` extension syntactically, but it just
 rejects all `foreign import javascript` declarations in the typechecker.

 Similar to the change in #11102 I think the proper thing to do is to make
 GHC say that `JavaScriptFFI` is not supported. Cabal 1.24 can then use
 this information in dependency resolution. GHCJS would then just add this
 to the list by itself, since it has its own front end.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11214>
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