Create 'type' annotation for 'const' variables

From https://developers.google.com/closure/compiler/docs/js-for-compiler:
  "The type declaration and additional comment are optional. If you
  provide a type declaration, put the declaration on a separate line."
Add 'const' annotation to ol.MapBrowserEvent.browserEvent
Remove pointless test
Add ol.source.ImageWMS tests
Add ol.source.TileWMS tests
Make ol.source.ImageSource crossOrigin property protected
Refactor ol.source.ImageWMS URL logic

* Always request images of integer WIDTH and HEIGHT
* Match BBOX to requested size
* Handle integer-only DPIs in GeoServer
Remove unused ol.source.wms.getDpiParam function
Merge pull request #1491 from fredj/const-annotation

Create 'type' annotation for 'const' variables
Add ol.source.ImageWMS#setUrl
Make ol.extent.buffer return an extent instead of mutating
Add ol.source.wms.DEFAULT_VERSION
Factor out WMS version 1.3 check
Refactor ol.source.TileWMS URL logic
Remove unused ol.source.wms.getUrl function
Remove unused ol.TileUrlFunction.createFromParamsFunction
Correct ol.renderer.canvas.ImageLayer transform for HiDPI devices
Merge remote-tracking branch 'openlayers/master' into vector-api
Merge pull request #1494 from twpayne/vector-api-wms-hidpi

[vector-api] HiDPI (Retina) support for WMS single image layers
Image source refactoring