This fixes an error when loading the diff viewer on release-5.0.x (and master). Devtools were showing an error that the `DiffViewerPage` constructor wasn't a constructor. The issue here seems to be a change in the way that the newer babel deals with function shorthands inside of an object. Whereas before it was translating `constructor() {...}` to `constructor: function constructor() {...}`, it now leaves them as-is. In most cases this works fine, but it seems like browsers are treating the `constructor` name specially (since it's a reserved word and used for classes). Explicitly using the older object attribute syntax fixes this. Testing Done: Created a review request with a diff and was able to successfully load the diff viewer page.