Check phone number formatting in the gallery text fields demo (#39450)

examples/flutter_gallery/test/demo/material/text_form_field_demo_test.dart

@@ -16,9 +16,17 @@ void main() {
     final Finder nameField = find.widgetWithText(TextFormField, 'Name *');
     expect(nameField, findsOneWidget);
 
+    final Finder phoneNumberField = find.widgetWithText(TextFormField, 'Phone Number *');
+    expect(phoneNumberField, findsOneWidget);
+
     final Finder passwordField = find.widgetWithText(TextFormField, 'Password *');
     expect(passwordField, findsOneWidget);
 
+    // Verify the that the phone number's TextInputFormatter does what's expected.
+    await tester.enterText(phoneNumberField, '1234567890');
+    await tester.pumpAndSettle();
+    expect(find.text('(123) 456-7890'), findsOneWidget);
+
     await tester.enterText(nameField, '');
     await tester.pumpAndSettle();
     // The submit button isn't initially visible. Drag it into view so that