use RadioListTile in expansion panels demo (flutter#20240)

* use RadioListTile in expansion panels demo

Fixes flutter#6048.
- expose `_Location`
- appease analyzer

Prevent dropdown menu's scroll offset from going negative (flutter#22235

)

In long lists this resulted in the dropdown scrolling to the very last
item in its list. Now clamping the value at `0.0`. Added a test to
verify that the selected item aligns with the button to test the offset.

Fixes flutter#15346

[Material] Add TabBarTheme (flutter#22012)

* Add tab bar theme.

* Add tab bar theme.

* Add tests, pass context to getters.

* update goldens from linux box

* update goldens from linux box

* Added new golden test, addressed comments

* override hashCode and == in TabBarTheme

* Fix comment typos

* Addressed Hans' comments.

* Formatting changes

* [TabBarTheme] Fixed spacing

* [TabBarTheme] Update goldens version to latest commit

Fix the run_release_test devicelab test after changing output. (flutt…

…er#22104)

Revert "Implement Double Tap Handling in TextField and Editable" (flu…

…tter#22051)

* Revert "Revert "Add RichText support to find.text" (flutter#22046)"

This reverts commit 8e70421.

* Revert "Implement Double Tap Handling in TextField and Editable (flutter#21264)"

This reverts commit 02e8733.

Make helper and error text separate widgets, make error and counter l…

…ive region (flutter#21752)

Use Xcode legacy build system for iOS builds (flutter#21901) (flutter…

…#21994)

Xcode 10 introduces a new build system which includes stricter checks on
duplicate build outputs.

When plugins are in use, there are two competing build actions that copy
Flutter.framework into the build application Frameworks directory:

  1. The Embed Frameworks build phase for the Runner project
  2. The [CP] Embed Pods Frameworks build phase that pod install creates
     in the project.

Item (1) is there to ensure the framework is copied into the built app
in the case where there are no plugins (and therefore no CocoaPods
integration in the Xcode project). Item (2) is there because Flutter's
podspec declares Flutter.framework as a vended_framework, and CocoaPods
automatically adds a copy step for each such vended_framework in the
transitive closure of CocoaPods dependencies.

As an immediate fix, we opt back into the build system used by Xcode 9
and earlier. Longer term, we need to update our templates and
flutter_tools to correctly handle this situation.

See: flutter#20685

Add back screenshot extension that was temporarily disabled. (flutter…

…#21828)

Add back screenshot extension that was temporarily disabled.

Add matchesReferenceImage matcher to test that the screenshot extension
generates equivalent images to InspectorService.instance.screenshot.

Remove all "flaky" annotations from devicelab (flutter#21895)

These tests don't seem particularly more flaky than ony others any more.

Roll engine b43b3e8..72665c6 (4 commits) (flutter#21916)

flutter/engine@b43b3e8...72665c6

git log b43b3e8..72665c6 --no-merges --oneline
72665c6 Roll src/third_party/skia 840debe9c748..098c1355bf70 (1 commits) (flutter/engine#6268)
d212e84 Roll src/third_party/skia 8e73f4b6bbef..840debe9c748 (1 commits) (flutter/engine#6267)
aaef8eb Roll src/third_party/skia afebe1614e5e..8e73f4b6bbef (1 commits) (flutter/engine#6265)
45696e7 Roll src/third_party/skia 7b617a7fc85d..afebe1614e5e (8 commits) (flutter/engine#6258)


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