Add a debug feature to the gestures library to dump hit test results (#11346)

packages/flutter/lib/gestures.dart

@@ -11,6 +11,7 @@ export 'src/gestures/arena.dart';
 export 'src/gestures/binding.dart';
 export 'src/gestures/constants.dart';
 export 'src/gestures/converter.dart';
+export 'src/gestures/debug.dart';
 export 'src/gestures/drag.dart';
 export 'src/gestures/drag_details.dart';
 export 'src/gestures/events.dart';

packages/flutter/lib/src/gestures/binding.dart

@@ -10,6 +10,7 @@ import 'package:flutter/foundation.dart';
 
 import 'arena.dart';
 import 'converter.dart';
+import 'debug.dart';
 import 'events.dart';
 import 'hit_test.dart';
 import 'pointer_router.dart';
@@ -75,6 +76,11 @@ abstract class GestureBinding extends BindingBase with HitTestable, HitTestDispa
       result = new HitTestResult();
       hitTest(result, event.position);
       _hitTests[event.pointer] = result;
+      assert(() {
+        if (debugPrintHitTestResults)
+          debugPrint('$event: $result');
+        return true;
+      });
     } else if (event is PointerUpEvent || event is PointerCancelEvent) {
       result = _hitTests.remove(event.pointer);
     } else if (event.down) {

packages/flutter/lib/src/gestures/debug.dart

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/foundation.dart';
+
+// Any changes to this file should be reflected in the debugAssertAllGesturesVarsUnset()
+// function below.
+
+/// Whether to print the results of each hit test to the console.
+///
+/// When this is set, in debug mode, any time a hit test is triggered by the
+/// [GestureBinding] the results are dumped to the console.
+///
+/// This has no effect in release builds.
+bool debugPrintHitTestResults = false;
+
+/// Returns true if none of the gestures library debug variables have been changed.
+///
+/// This function is used by the test framework to ensure that debug variables
+/// haven't been inadvertently changed.
+///
+/// See [https://docs.flutter.io/flutter/gestures/gestures-library.html] for
+/// a complete list.
+bool debugAssertAllGesturesVarsUnset(String reason) {
+  assert(() {
+    if (debugPrintHitTestResults)
+      throw new FlutterError(reason);
+    return true;
+  });
+  return true;
+}

packages/flutter_test/lib/src/binding.dart

@@ -474,6 +474,9 @@ abstract class TestWidgetsFlutterBinding extends BindingBase
       'The value of a foundation debug variable was changed by the test.',
       debugPrintOverride: debugPrintOverride,
     ));
+    assert(debugAssertAllGesturesVarsUnset(
+      'The value of a gestures debug variable was changed by the test.',
+    ));
     assert(debugAssertAllRenderVarsUnset(
       'The value of a rendering debug variable was changed by the test.',
       debugCheckIntrinsicSizesOverride: checkIntrinsicSizes,