Fix flutter_gallery_instrumentation_test (#20361)

Somehow I forgot to say "super.tap()" when calling "tap()" on the new
superclass, so it was just recursing infinitely but ended up actually
crashing on the first reuse of the finder.

The error was previously swallowed, I made this print it instead.

dev/devicelab/bin/tasks/flutter_gallery_instrumentation_test.dart

@@ -9,6 +9,10 @@ import 'package:flutter_devicelab/framework/adb.dart';
 import 'package:flutter_devicelab/framework/framework.dart';
 import 'package:flutter_devicelab/framework/utils.dart';
 
+// This test runs "//examples/flutter_gallery/test/live_smoketest.dart", which communicates
+// with the Java code to report its status. If this test fails due to a problem on the Dart
+// side, you can debug that by just running that file directly using `flutter run`.
+
 Future<Null> main() async {
   deviceOperatingSystem = DeviceOperatingSystem.android;
 

examples/flutter_gallery/test/live_smoketest.dart

@@ -2,6 +2,12 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// ATTENTION!
+//
+// This file is not named "*_test.dart", and as such will not run when you run
+// "flutter test". It is only intended to be run as part of the
+// flutter_gallery_instrumentation_test devicelab test.
+
 import 'dart:async';
 
 import 'package:flutter/services.dart';
@@ -43,32 +49,34 @@ Future<Null> main() async {
     if (!new Set<String>.from(allDemoTitles).containsAll(_kSkippedDemoTitles))
       fail('Unrecognized demo names in _kSkippedDemoTitles: $_kSkippedDemoTitles');
 
+    print('Starting app...');
     runApp(const GalleryApp(testMode: true));
     final _LiveWidgetController controller = new _LiveWidgetController(WidgetsBinding.instance);
     for (GalleryDemoCategory category in kAllGalleryDemoCategories) {
+      print('Tapping "${category.name}" section...');
       await controller.tap(find.text(category.name));
       for (GalleryDemo demo in kGalleryCategoryToDemos[category]) {
         final Finder demoItem = find.text(demo.title);
+        print('Scrolling to "${demo.title}"...');
         await controller.scrollIntoView(demoItem, alignment: 0.5);
-
-        if (_kSkippedDemoTitles.contains(demo.title)) {
-          print('> skipped $demo');
+        if (_kSkippedDemoTitles.contains(demo.title))
           continue;
-        }
-
         for (int i = 0; i < 2; i += 1) {
+          print('Tapping "${demo.title}"...');
           await controller.tap(demoItem); // Launch the demo
           controller.frameSync = !_kUnsynchronizedDemoTitles.contains(demo.title);
+          print('Going back to demo list...');
           await controller.tap(find.byTooltip('Back'));
           controller.frameSync = true;
         }
-        print('Success');
       }
+      print('Going back to home screen...');
       await controller.tap(find.byTooltip('Back'));
     }
-
+    print('Finished successfully!');
     _kTestChannel.invokeMethod('success');
-  } catch (error) {
+  } catch (error, stack) {
+    print('Caught error: $error\n$stack');
     _kTestChannel.invokeMethod('failure');
   }
 }
@@ -96,19 +104,16 @@ class _LiveWidgetController extends LiveWidgetController {
   /// Runs `finder` repeatedly until it finds one or more [Element]s.
   Future<Finder> _waitForElement(Finder finder) async {
     if (frameSync)
-      await _waitUntilFrame(() => SchedulerBinding.instance.transientCallbackCount == 0);
-
+      await _waitUntilFrame(() => binding.transientCallbackCount == 0);
     await _waitUntilFrame(() => finder.precache());
-
     if (frameSync)
-      await _waitUntilFrame(() => SchedulerBinding.instance.transientCallbackCount == 0);
-
+      await _waitUntilFrame(() => binding.transientCallbackCount == 0);
     return finder;
   }
 
   @override
   Future<Null> tap(Finder finder, { int pointer }) async {
-    await tap(await _waitForElement(finder), pointer: pointer);
+    await super.tap(await _waitForElement(finder), pointer: pointer);
   }
 
   Future<Null> scrollIntoView(Finder finder, {double alignment}) async {