Fix test that leaks images.

[polina-c]

Contributes to #145599

Repro:
flutter test test/cupertino/tab_scaffold_test.dart --dart-define LEAK_TRACKING=true --plain-name "Adding new tabs does not crash the app"

What is going on:

1. ImageCache.putIfAbsent, in case the image already existed in the cache, invokes ImageCache._trackLiveImage, that creates _LiveImage and passes the image's completer as parameter completer
2. _LiveImage constructor invokes super constructor (of _CachedImageBase) that initializes the member handle
3. handle is disposed using scheduler, and disposal does not happen in time of test completion.

Adding delay to the test increases number of not disposed objects from 30 to 120.
Should we force schedule at the end of the widget tests somehow to make scheduler switched to right state?

Creation call stack:

#9______new_ImageStreamCompleterHandle.__(package:flutter/src/painting/image_stream.dart:465:41)
#10_____ImageStreamCompleter.keepAlive_(package:flutter/src/painting/image_stream.dart:655:39)
#11_____new__CachedImageBase_(package:flutter/src/painting/image_cache.dart:609:27)
#12_____new__LiveImage_(package:flutter/src/painting/image_cache.dart:647:9)
#13_____ImageCache._trackLiveImage.<anonymous_closure>_(package:flutter/src/painting/image_cache.dart:302:14)
#14______LinkedHashMapMixin.putIfAbsent_(dart:collection-patch/compact_hash.dart:543:23)
#15_____ImageCache._trackLiveImage_(package:flutter/src/painting/image_cache.dart:296:17)
#16_____ImageCache.putIfAbsent_(package:flutter/src/painting/image_cache.dart:378:7)
#17_____ImageProvider.resolveStreamForKey_(package:flutter/src/painting/image_provider.dart:517:81)
                      #18_____ScrollAwareImageProvider.resolveStreamForKey_(package:flutter/src/widgets/scroll_aware_image_provider.dart:104:19)
#19_____ImageProvider.resolve.<anonymous_closure>_(package:flutter/src/painting/image_provider.dart:366:9)
#20_____ImageProvider._createErrorHandlerAndKey.<anonymous_closure>_(package:flutter/src/painting/image_provider.dart:479:24)
#21_____SynchronousFuture.then_(package:flutter/src/foundation/synchronous_future.dart:43:39)
                      #22_____ImageProvider._createErrorHandlerAndKey_(package:flutter/src/painting/image_provider.dart:476:9)
#23_____ImageProvider.resolve_(package:flutter/src/painting/image_provider.dart:363:5)
#24______ImageState._resolveImage_(package:flutter/src/widgets/image.dart:1111:16)
#25______ImageState.didChangeDependencies_(package:flutter/src/widgets/image.dart:1061:5)
#26_____StatefulElement._firstBuild_(package:flutter/src/widgets/framework.dart:5630:11)
#27_____ComponentElement.mount_(package:flutter/src/widgets/framework.dart:5457:5)
#28_____Element.inflateWidget_(package:flutter/src/widgets/framework.dart:4334:16)
#29_____Element.updateChild_(package:flutter/src/widgets/framework.dart:3843:18)
#30_____SingleChildRenderObjectElement.mount_(package:flutter/src/widgets/framework.dart:6763:14)

[Hixie]

replacing this line with imageCache.clear(); fixes all the leaks in this test for me.

[polina-c]

On my mac with imageCache.clear(); there are less leaks with this change and they are different:

  Expected: leak free
    Actual: <Instance of 'Leaks'>
     Which: contains leaks:
            # The text is generated by leak_tracker.
            # For leak troubleshooting tips open:
            # https://github.com/dart-lang/leak_tracker/blob/main/doc/TROUBLESHOOT.md
            notDisposed:
              total: 6
              objects:
                StatefulElement:
                  test: Adding new tabs does not crash the app
                  identityHashCode: 823867307
                _MediaQueryFromViewState:
                  test: Adding new tabs does not crash the app
                  identityHashCode: 701933900
                LeafRenderObjectElement:
                  test: Adding new tabs does not crash the app
                  identityHashCode: 637995193
                RenderErrorBox:
                  test: Adding new tabs does not crash the app
                  identityHashCode: 761576866
                PictureLayer:
                  test: Adding new tabs does not crash the app
                  identityHashCode: 1042115368
                _NativePicture:
                  test: Adding new tabs does not crash the app
                  identityHashCode: 515013832

But bots run on Windows, and if on Windows there are no leaks, it is good fix for now. Eventually we will add bots for other platforms.

However there is a question:
There are many tests that deal with animation. Why they are not leaking? Why we need to clean cash just for this specific test? I think we need explanation to guide developers when it is needed.

[Hixie]

This test loads images. The images get cached. The cache is shared across tests (intentionally). My theory about it being related to animated GIFs was mistaken.

[Hixie]

We should document somewhere that if you use cached images, they'll get flagged as leaked unless you manually clear the cache.

[Hixie]

Per https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo#make-each-test-entirely-self-contained it's generally better to put this in each test, that way you can easily copy tests to other files and generally easily tell what exactly each test is doing.

[polina-c]

Clean up for each test will improve test isolation.
Clean up just at the end will improve test performance.

I guess one of the reasons image cashing exists is exactly performance of testing.

Existing code uses tearDown. So, switched.

[Hixie]

cc @jonahwilliams who is probably the person most familiar with the image cache?
cc @goderbauer and @Piinks who may have opinions too

[polina-c]

We should document somewhere that if you use cached images, they'll get flagged as leaked unless you manually clear the cache.

Added the guidance to the page that is linked in leak related exceptions: dart-lang/leak_tracker#233

[jonahwilliams]

LGTM