Make tester.startGesture less async, for better stack traces

[gnprice]

Toward #85160.

When tests interfere with each other over gestures, debugging the problem can get confusing. One reason is that the stack traces are obscured by asynchrony, in a way that turns out to have an easy fix. This PR makes that fix.

---

For example, here's a stack trace I encountered when debugging one of the state leaks in widgets/draggable_test.dart:

══╡ EXCEPTION CAUGHT BY FLUTTER TEST FRAMEWORK ╞════════════════════════════════════════════════════
The following assertion was thrown running a test:
Pointer of PointerDownEvent#21bfd(position: Offset(400.0, 72.0), pointer: 7, kind: touch, buttons:
1, down: true) unexpectedly has a HitTestResult associated with it.
'package:flutter/src/gestures/binding.dart':
Failed assertion: line 369 pos 14: '!_hitTests.containsKey(event.pointer)'

Either the assertion indicates an error in the framework itself, or we should provide substantially
more information in this error message to help you determine and fix the underlying cause.
In either case, please report this assertion by filing a bug on GitHub:
  https://github.com/flutter/flutter/issues/new?template=2_bug.md

When the exception was thrown, this was the stack:
#2      GestureBinding._handlePointerEventImmediately (package:flutter/src/gestures/binding.dart:369:14)
#3      GestureBinding.handlePointerEvent (package:flutter/src/gestures/binding.dart:363:5)
#4      TestWidgetsFlutterBinding.handlePointerEventForSource.<anonymous closure> (package:flutter_test/src/binding.dart:614:42)
#5      TestWidgetsFlutterBinding.withPointerEventSource (package:flutter_test/src/binding.dart:624:11)
#6      TestWidgetsFlutterBinding.handlePointerEventForSource (package:flutter_test/src/binding.dart:614:5)
#7      WidgetTester.sendEventToBinding.<anonymous closure> (package:flutter_test/src/widget_tester.dart:849:15)
#10     TestAsyncUtils.guard (package:flutter_test/src/test_async_utils.dart:68:41)
#11     WidgetTester.sendEventToBinding (package:flutter_test/src/widget_tester.dart:848:27)
#12     TestGesture.down.<anonymous closure> (package:flutter_test/src/test_pointer.dart:456:25)
#15     TestAsyncUtils.guard (package:flutter_test/src/test_async_utils.dart:68:41)
#16     TestGesture.down (package:flutter_test/src/test_pointer.dart:455:27)
#17     WidgetController.startGesture (package:flutter_test/src/controller.dart:1183:20)
<asynchronous suspension>
<asynchronous suspension>
(elided 7 frames from class _AssertionError, dart:async, and package:stack_trace)

The test description was:
  Drag and drop - dragging over button
════════════════════════════════════════════════════════════════════════════════════════════════════

That test, "Drag and drop - dragging over button", is 90 lines long. So the first thing one wants to know to debug it is where in the test the error occurred — what steps have already happened to get us to the state where the assertion detected a problem, and what step we were on that triggered the assert.

And unfortunately this stack trace doesn't tell us that; it doesn't contain any frames from the test. The outermost frame is in WidgetController.startGesture, inside the flutter_test library.

The test does call that method, though — on line 451 and line 467. So one might guess that one of those calls triggered the failure.

That turns out to be the wrong track.

After applying this PR, here's the stack trace from the same debugging attempt:

When the exception was thrown, this was the stack:
#2      GestureBinding._handlePointerEventImmediately (package:flutter/src/gestures/binding.dart:369:14)
#3      GestureBinding.handlePointerEvent (package:flutter/src/gestures/binding.dart:363:5)
#4      TestWidgetsFlutterBinding.handlePointerEventForSource.<anonymous closure> (package:flutter_test/src/binding.dart:614:42)
#5      TestWidgetsFlutterBinding.withPointerEventSource (package:flutter_test/src/binding.dart:624:11)
#6      TestWidgetsFlutterBinding.handlePointerEventForSource (package:flutter_test/src/binding.dart:614:5)
#7      WidgetTester.sendEventToBinding.<anonymous closure> (package:flutter_test/src/widget_tester.dart:849:15)
#10     TestAsyncUtils.guard (package:flutter_test/src/test_async_utils.dart:68:41)
#11     WidgetTester.sendEventToBinding (package:flutter_test/src/widget_tester.dart:848:27)
#12     TestGesture.down.<anonymous closure> (package:flutter_test/src/test_pointer.dart:456:25)
#15     TestAsyncUtils.guard (package:flutter_test/src/test_async_utils.dart:68:41)
#16     TestGesture.down (package:flutter_test/src/test_pointer.dart:455:27)
#17     WidgetController.startGesture (package:flutter_test/src/controller.dart:1187:20)
#18     WidgetController.tapAt.<anonymous closure> (package:flutter_test/src/controller.dart:560:41)
#21     TestAsyncUtils.guard (package:flutter_test/src/test_async_utils.dart:68:41)
#22     WidgetController.tapAt (package:flutter_test/src/controller.dart:559:27)
#23     WidgetController.tap (package:flutter_test/src/controller.dart:554:12)
#24     main.<anonymous closure> (file:///home/greg/n/flutter/flutter/packages/flutter/test/widgets/draggable_test.dart:476:18)
<asynchronous suspension>
<asynchronous suspension>
(elided 9 frames from class _AssertionError, dart:async, and package:stack_trace)

This immediately tells us the error is happening on line 476 of the test file. That's several steps later than the tester.startGesture calls; it's instead a call to tester.tap. The startGesture call on the stack was an internal one.

With that information in hand, we avoid some fruitless paths of debugging, and can instead go straight to investigating why that particular tap call found itself in this situation.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat (don't just cc him here, he won't see it! He's on Discord!).

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[gnprice]

Test added.

[chunhtai]

LGTM