Fix Banner onDismiss not being called

Author: lemonmade

src/components/Banner/Banner.tsx

@@ -114,7 +114,12 @@ export default function Banner({
   const dismissButton = onDismiss
     ? (
       <div className={styles.Dismiss}>
-        <Button plain icon="cancelSmall" accessibilityLabel="Dismiss notification" />
+        <Button
+          plain
+          icon="cancelSmall"
+          onClick={onDismiss}
+          accessibilityLabel="Dismiss notification"
+        />
       </div>
     )
     : null;

src/components/Banner/tests/Banner.test.tsx

@@ -0,0 +1,15 @@
+import * as React from 'react';
+import {shallow} from 'enzyme';
+import Banner from '..';
+import Button from '../../Button';
+
+describe('<Banner />', () => {
+  describe('onDismiss()', () => {
+    it('is called when the dismiss button is clicked', () => {
+      const spy = jest.fn();
+      const banner = shallow(<Banner onDismiss={spy} />);
+      banner.find(Button).simulate('click');
+      expect(spy).toHaveBeenCalled();
+    });
+  });
+});