Explicitly define type of 'this'

I'm not entirely sure that I like this pattern, but there does not
appear to be a better option since TypeScript would recommend:

  function(this: SomeClass, ...)

tsdoc does not appear to support an associated annotation for the
function a la @this {SomeClass}.

src/ol/Tile.js

@@ -180,7 +180,7 @@ class Tile extends EventTarget {
     }
 
     let tile = this.interimTile;
-    let prev = this;
+    let prev = /** @type {Tile} */ (this);
 
     do {
       if (tile.getState() == TileState.LOADED) {