This converts the JavaScript unit tests for RB.DiffViewerPageView and

RB.ReviewablePageView to ES6, in preparation for a new set of tests

coming in.
The tests for RB.ReviewRequestPageView have been fixed to use

function() {} instead of () => {} for Jasmine tests, since the

former is recommended (due to Jasmine populating this with state, and

the ability for test suites to store their own local state in Jasmine's

this). It also fixes a variable name (page to pageView), which

will help with future tests.

All unit tests pass.