Underline assertEqual line when it fails

Summary:
Got feedback from reviewer that she didn't notice gutter icon,
so this adds an underline to the relevant line of code.
If the assertion spans 2 lines, the second wont be highlighted,
but am okay with that.

Test Plan: - Write Program.assertEqual(2, 4);, see line show up

Reviewers: alexrodrigues, john

Reviewed By: john

Subscribers: john, #cs

Differential Revision: http://phabricator.khanacademy.org/D14041