* Unfortunately this on its own does not work yet due to Sprockets
  recursively calling Sprockets::CachedEnvironment#load.
* Reproduce the recursion issue with:
  bundle exec bin/sprockets -I test/fixtures/asset $PWD/test/fixtures/asset/application.js

* This does not hold any lock while computing the missing key,
  so avoids the problem of having a lock while recursing when
  using #compute_if_absent.
* As the docs say:
  https://ruby-concurrency.github.io/concurrent-ruby/master/Concurrent/Map.html#fetch_or_store-instance_method
  "The store can overwrite other concurrently stored value.".
  Which was already possibly before, Hash#fetch does not have the
  guarantee to execute the block only once per key, even on CRuby.

* This should help catching concurrency issues much faster.
* Notably, the two issues of rails#772 reproduce by running the test suite on TruffleRuby.
* Skip tests which do not pass yet on TrufffleRuby.
  oracle/truffleruby#2810

* host is "" instead of nil, following the upstream change:
  ruby/uri@81263c9