Added element extensions spec for .matches and .closest

spec/javascripts/extensions/element_spec.js.es6

+/*= require extensions/element */
+
+(() => {
+  describe('Element extensions', function () {
+    beforeEach(() => {
+      this.element = document.createElement('ul');
+    });
+
+    describe('matches', () => {
+      it('returns true if element matches the selector', () => {
+        expect(this.element.matches('ul')).toBeTruthy();
+      });
+
+      it("returns false if element doesn't match the selector", () => {
+        expect(this.element.matches('.not-an-element')).toBeFalsy();
+      });
+    });
+
+    describe('closest', () => {
+      beforeEach(() => {
+        this.childElement = document.createElement('li');
+        this.element.appendChild(this.childElement);
+      });
+
+      it('returns the closest parent that matches the selector', () => {
+        expect(this.childElement.closest('ul').toString()).toBe(this.element.toString());
+      });
+
+      it('returns itself if it matches the selector', () => {
+        expect(this.childElement.closest('li').toString()).toBe(this.childElement.toString());
+      });
+
+      it('returns undefined if nothing matches the selector', () => {
+        expect(this.childElement.closest('.no-an-element')).toBeFalsy();
+      });
+    });
+  });
+})();