Actually this file may be invoked directly as well with --dev.

railties/lib/rails/commands/application.rb

@@ -12,7 +12,6 @@
 end
 
 require 'rubygems' if ARGV.include?("--dev")
-
 require 'rails/generators'
 require 'rails/generators/rails/app/app_generator'
 

railties/lib/rails/commands/plugin_new.rb

+require 'rubygems' if ARGV.include?("--dev")
+
 if ARGV.first != "new"
   ARGV[0] = "--help"
 else
   ARGV.shift
 end
 
+require 'rails/generators'
 require 'rails/generators/rails/plugin_new/plugin_new_generator'
 Rails::Generators::PluginNewGenerator.start
\ No newline at end of file