add an assertion message.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_9_2@28276 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
merges r28174 from trunk into ruby_1_9_2.¶
add an assertion message.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_9_2@28276 b2dd03c8-39d4-4d8f-98ff-823fe69b080e