[RubyGems] Allow standalone caching of builtin gems

lib/bundler/installer.rb

@@ -102,13 +102,15 @@ def install_gem_from_spec(spec, standalone = false, worker = 0, force = false)
       settings = Bundler.settings["build.#{spec.name}"]
       messages = nil
 
+      install_options = { :force => force, :ensure_builtin_gems_cached => standalone }
+
       if settings
         # Build arguments are global, so this is mutexed
         Bundler.rubygems.with_build_args [settings] do
-          messages = spec.source.install(spec, force)
+          messages = spec.source.install(spec, install_options)
         end
       else
-        messages = spec.source.install(spec, force)
+        messages = spec.source.install(spec, install_options)
       end
 
       install_message, post_install_message, debug_message = *messages

lib/bundler/source/rubygems.rb

@@ -85,9 +85,22 @@ def specs
         end
       end
 
-      def install(spec, force = false)
+      def install(spec, opts = {})
+        force = opts[:force]
+        ensure_builtin_gems_cached = opts[:ensure_builtin_gems_cached]
+
+        if ensure_builtin_gems_cached && builtin_gem?(spec)
+          if !cached_path(spec)
+            cached_built_in_gem(spec) unless spec.remote
+            force = true
+          else
+            spec.loaded_from = loaded_from(spec)
+          end
+        end
+
         return ["Using #{version_message(spec)}", nil] if installed_specs[spec].any? && !force
 
+
         # Download the gem to get the spec, because some specs that are returned
         # by rubygems.org are broken and wrong.
         if spec.remote