rails 5.1.1，ruby 2.4.1，webpacker 2.0，运行系统测试通过rake test:system。
如果有办法查看更多的调试日志，将有帮助
请注意，只有在没有缓存的情况下才会发生，该测试应进行。如果有缓存，WebPacker根本不运行，这是正常的。
/usr/local/lib/ruby/gems/2.4.0/gems/webpacker-2.0/lib/tasks/installers.rake:1: warning: already initialized constant INSTALLERS
/usr/local/lib/ruby/gems/2.4.0/gems/webpacker-2.0/lib/tasks/installers.rake:1: warning: previous definition of INSTALLERS was here
/usr/local/lib/ruby/gems/2.4.0/gems/webpacker-2.0/lib/tasks/webpacker/install.rake:1: warning: already initialized constant WEBPACKER_APP_TEMPLATE_PATH
/usr/local/lib/ruby/gems/2.4.0/gems/webpacker-2.0/lib/tasks/webpacker/install.rake:1: warning: previous definition of WEBPACKER_APP_TEMPLATE_PATH was here
Webpacker is installed � �
Using ..../config/webpacker.yml file for setting up webpack paths
Webpacker is installed � �
Using ..../config/webpacker.yml file for setting up webpack paths
[Webpacker] Compiling assets �
[Webpacker] Compiling assets �
事实证明，webpacker的耙子任务再次加载rakefile。
https://github.com/rails/webpacker/pull/477/files