Skip the gallery smoke test (#3736)

This test hangs occationally on the bots and the bots aren't smart enough to
recover, which means the whole project gets blocked.

Filed #3735 about the hang.

examples/material_gallery/test/smoke_test.dart

@@ -94,5 +94,5 @@ void main() {
     tester.tap(find.text('Light'));
     tester.pump();
     tester.pump(const Duration(seconds: 1)); // Wait until it's changed.
-  });
+  }, skip: true);
 }

packages/flutter_test/lib/src/widget_tester.dart

@@ -36,13 +36,14 @@ typedef Future<Null> WidgetTesterCallback(WidgetTester widgetTester);
 ///       expect(tester, hasWidget(find.text('Success')));
 ///     });
 void testWidgets(String description, WidgetTesterCallback callback, {
-  Timeout timeout: const Timeout(const Duration(seconds: 5))
+  Timeout timeout: const Timeout(const Duration(seconds: 5)),
+  bool skip
 }) {
   TestWidgetsFlutterBinding binding = TestWidgetsFlutterBinding.ensureInitialized();
   WidgetTester tester = new WidgetTester._(binding);
   group('-', () {
     setUp(binding.preTest);
-    test(description, () => binding.runTest(() => callback(tester)));
+    test(description, () => binding.runTest(() => callback(tester)), skip: skip);
     tearDown(binding.postTest);
   }, timeout: timeout);
 }