Fix ReorderableListView's use of child keys (#41334)

[krisgiesing]

Description

ReorderableListView was constructing a GlobalObjectKey using the child key as the value. This only had the intended behavior if the child key was identical across build method invocations.

The new strategy is to scope the child key's value to the State object's identity, allowing child keys to have value compare semantics while disambiguating among different list view instances.

Related Issues

#41334 - fixed

Tests

Updated test/material/reorderable_list_test.dart to add 'Preserves children states across reorder when keys are not identical'.

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It looks like this pull request may not have tests. Please make sure to add tests before merging. While there are exceptions to this rule, if this patch modifies code it is probably not an exception.

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[krisgiesing]

@Hixie ready for review, I think.

[Hixie]

This is great.

[Hixie]

Some analyzer warnings but otherwise LGTM.

[slightfoot]

@Hixie Looks like this change has broken something. This is gallery ReorderableList demo: https://github.com/flutter/flutter/blob/master/examples/flutter_gallery/lib/demo/material/reorderable_list_demo.dart

Flutter 1.10.7 • channel dev • https://github.com/flutter/flutter.git
Framework • revision e70236e36c (27 hours ago) • 2019-10-02 09:32:30 -0700
Engine • revision 9e6314d348
Tools • Dart 2.6.0 (build 2.6.0-dev.4.0 1103600280)

I am experiencing the same issue in my current app I am building.

[Hixie]

cc @krisgiesing can you revert while the issue is being investigated?

I wonder what's going on...

[krisgiesing]

The issue preventing this from landing has been narrowed down to a test that doesn't use ReorderableListView, captured in #43780.