support for iOS 11

e2e/TopLevelApi.test.js

   it('unmount is called on pop', async () => {
     await elementByLabel('Push Lifecycle Screen').tap();
     await expect(elementByLabel('didAppear')).toBeVisible();
-    await element(by.traits(['button']).and(by.label('Back'))).tap();
+    try {
+      await element(by.trait(['button']).and(by.label('Back'))).tap();
+    } catch (err) {
+      await element(by.type('_UIModernBarButton').and(by.label('Back'))).tap();
+    }
     await expect(elementByLabel('didDisappear')).toBeVisible();
     await expect(elementByLabel('componentWillUnmount')).toBeVisible();
   });