The current diff viewer shows the changes for all files and it can be 

tedious to scroll through, especially with large changes. Adding the 

ability to hide/unhide files will better allow users to look at only 

a subset of the changes at once.
We added a toggle to the header for each file that collapses the file 

to a single line filename. Toggling a collapsed file header will expand 

the entire file. This feature is referenced from the toggling of 

Reviews in a Review Request. When the toggle button is clicked, 

jQuery toggle() is called on the revision-row class and tbody 

attribute. The toggle() function works toggling the CSS display 

property between display: none.

Wrote onToggleCollapseClicked in diffReviewableViewTests.es6.js.