FIX: Don't add constants for ES6 modules in plugins

If you're using ES6 in a plugin, you shouldn't be depending on the
backwards compatibility layer. That's for proper Discourse only.

lib/es6_module_transpiler/tilt/es6_module_transpiler_template.rb

@@ -70,7 +70,9 @@ module Tilt
       # For backwards compatibility with plugins, for now export the Global format too.
       # We should eventually have an upgrade system for plugins to use ES6 or some other
       # resolve based API.
-      if ENV['DISCOURSE_NO_CONSTANTS'].nil? && scope.logical_path =~ /(discourse|admin)\/(controllers|components|views|routes|mixins|models)\/(.*)/
+      if ENV['DISCOURSE_NO_CONSTANTS'].nil? &&
+        scope.logical_path =~ /(discourse|admin)\/(controllers|components|views|routes|mixins|models)\/(.*)/
+
         type = Regexp.last_match[2]
         file_name = Regexp.last_match[3].gsub(/[\-\/]/, '_')
         class_name = file_name.classify
@@ -81,7 +83,7 @@ module Tilt
         end
         require_name = module_name(scope.root_path, scope.logical_path)
 
-        if require_name !~ /\-test$/
+        if require_name !~ /\-test$/ && require_name !~ /^discourse\/plugins\//
           result = "#{class_name}#{type.classify}"
 
           # HAX