Fix initializing the HscEnv for loading plugins:   - strip the 'js_' prefix from the hi file extensions   - use initDynFlags to prevent the plugins HscEnv from interfering        with the JavaScript HscEnv through the mutable parts of        DynFlags
Fix initializing the HscEnv for loading plugins:
  - strip the 'js_' prefix from the hi file extensions
  - use initDynFlags to prevent the plugins HscEnv from interfering
       with the JavaScript HscEnv through the mutable parts of
       DynFlags

ghcjs/ghcjs,ghcjs/ghcjs,ghcjs/ghcjs,ghcjs/ghcjs,ghcjs/ghcjs,ghcjs/ghcjs,ghcjs/ghcjs