Use blocked scoped variables

In addition to using const and let, this also upgrades our linter config and removes lint (mostly whitespace).

test/spec/ol/overlay.test.js

@@ -4,15 +4,15 @@ import View from '../../../src/ol/View.js';
 
 
 describe('ol.Overlay', function() {
-  var target, map;
+  let target, map;
 
-  var width = 360;
-  var height = 180;
+  const width = 360;
+  const height = 180;
 
   beforeEach(function() {
     target = document.createElement('div');
 
-    var style = target.style;
+    const style = target.style;
     style.position = 'absolute';
     style.left = '-1000px';
     style.top = '-1000px';
@@ -38,12 +38,12 @@ describe('ol.Overlay', function() {
   describe('constructor', function() {
 
     it('can be constructed with minimal arguments', function() {
-      var instance = new Overlay({});
+      const instance = new Overlay({});
       expect(instance).to.be.an(Overlay);
     });
 
     it('can be constructed with className', function() {
-      var instance = new Overlay({className: 'my-class'});
+      const instance = new Overlay({className: 'my-class'});
       expect(instance).to.be.an(Overlay);
       expect(instance.element.className).to.be('my-class');
     });
@@ -51,7 +51,7 @@ describe('ol.Overlay', function() {
   });
 
   describe('#getId()', function() {
-    var overlay, target;
+    let overlay, target;
 
     beforeEach(function() {
       target = document.createElement('div');
@@ -80,7 +80,7 @@ describe('ol.Overlay', function() {
   });
 
   describe('#setVisible()', function() {
-    var overlay, target;
+    let overlay, target;
 
     beforeEach(function() {
       target = document.createElement('div');