Move quadKey function to the one place it is used

src/ol/source/BingMaps.js

@@ -8,10 +8,35 @@ import {jsonp as requestJSONP} from '../net.js';
 import {get as getProjection, getTransformFromProjections} from '../proj.js';
 import SourceState from './State.js';
 import TileImage from './TileImage.js';
-import {createOrUpdate, quadKey} from '../tilecoord.js';
+import {createOrUpdate} from '../tilecoord.js';
 import {createXYZ, extentFromProjection} from '../tilegrid.js';
 
 
+/**
+ * @param {import('../tilecoord.js').TileCoord} tileCoord Tile coord.
+ * @return {string} Quad key.
+ */
+export function quadKey(tileCoord) {
+  const z = tileCoord[0];
+  const digits = new Array(z);
+  let mask = 1 << (z - 1);
+  let i, charCode;
+  for (i = 0; i < z; ++i) {
+    // 48 is charCode for 0 - '0'.charCodeAt(0)
+    charCode = 48;
+    if (tileCoord[1] & mask) {
+      charCode += 1;
+    }
+    if (tileCoord[2] & mask) {
+      charCode += 2;
+    }
+    digits[i] = String.fromCharCode(charCode);
+    mask >>= 1;
+  }
+  return digits.join('');
+}
+
+
 /**
  * The attribution containing a link to the Microsoft® Bing™ Maps Platform APIs’
  * Terms Of Use.
@@ -209,6 +234,7 @@ class BingMaps extends TileImage {
     const hidpi = this.hidpi_;
     this.tileUrlFunction = createFromTileUrlFunctions(
       resource.imageUrlSubdomains.map(function(subdomain) {
+        /** @type {import('../tilecoord.js').TileCoord} */
         const quadKeyTileCoord = [0, 0, 0];
         const imageUrl = resource.imageUrl
           .replace('{subdomain}', subdomain)