# File lib/active_support/testing/assertions.rb, line 34
      def assert_difference(expression, difference = 1, message = nil, &block)
        b = block.send(:binding)
        exps = Array.wrap(expression)
        before = exps.map { |e| eval(e, b) }

        yield

        exps.each_with_index do |e, i|
          error = "#{e.inspect} didn't change by #{difference}"
          error = "#{message}.\n#{error}" if message
          assert_equal(before[i] + difference, eval(e, b), error)
        end
      end