Normalize assert checking of clipBehavior (flutter#38568)

I noticed that we were pretty inconsistent with the way that we checked the value of clipBehavior in the framework, so I normalized the usages and updated docs where necessary.

This is a breaking change if you used to pass null explicitly to FlatButton, OutlineButton or RaisedButton constructors, expecting to get Clip.none. It will now assert if you do that. Existing implementations that pass null implicitly by not specifying clipBehavior won't need to change their call sites. It always implicitly defaulted to Clip.none before, and it will continue to do that, it's only places where it was explicitly set to null in order to get the implicit default that it will fail.

Correct InheritedTheme.captureAll() for multiple theme ancestors of t…

…he same type (flutter#39089)

Revert "Automatic focus highlight mode for FocusManager (flutter#37825)…

…" (flutter#38866)

This reverts commit a11d731 because of a regression in
flutter_gallery_ios32__transition_perf's 90th_percentile_frame_build_time_millis.

Fixes flutter#38860.

Roll engine 21ae926..3e10d92 (7 commits) (flutter#38855)

[email protected]:flutter/engine.git/compare/21ae92651a38...3e10d92

git log 21ae926..3e10d92 --no-merges --oneline
2019-08-19 [email protected] Roll src/third_party/dart 0e201edeeb..9f13d07670 (27 commits)
2019-08-19 [email protected] Notify framework to clear input connection when app is backgrounded (flutter#35054) (flutter/engine#9498)
2019-08-19 [email protected] Re-enable firebase test and don't use google login (flutter/engine#11228)
2019-08-19 [email protected] Update tflite_native and language_model revisions to match the Dart SDK (flutter/engine#11230)
2019-08-19 [email protected] Roll src/third_party/skia f8221786d088..8566dda51b42 (4 commits) (flutter/engine#11220)
2019-08-19 [email protected] Update metal layer drawable size on relayout. (flutter/engine#11224)
2019-08-19 [email protected] Make firebase testlab always pass (flutter/engine#11226)

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Roll engine 3dc233f..5540684 (5 commits) (flutter#38756)

[email protected]:flutter/engine.git/compare/3dc233f04e8e...5540684

git log 3dc233f..5540684 --no-merges --oneline
2019-08-17 [email protected] Roll src/third_party/skia 9318a6cfbcdd..4b567cd679ab (1 commits) (flutter/engine#11077)
2019-08-17 [email protected] [dynamic_thread_merging] Resubmit only on the frame where the merge (flutter/engine#11075)
2019-08-17 [email protected] Roll fuchsia/clang/mac-amd64 from VdFWD... to 4uAyd... (flutter/engine#11074)
2019-08-17 [email protected] Roll src/third_party/skia a0c9d43c2075..9318a6cfbcdd (1 commits) (flutter/engine#11073)
2019-08-17 [email protected] Roll src/third_party/dart beee442625..79e6c74337 (8 commits)

The AutoRoll server is located here: https://autoroll.skia.org/r/flutter-engine-flutter-autoroll

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[linux] Receives the unmodified characters obtained from GLFW (flutte…

…r#34752)

Hide text selection handle after entering text (flutter#37436)

The text caret wasn't being hidden after entering text, this fixes it.

use SizedBox instead of Container for building collapsed selection (f…

…lutter#37048)

Cherry-picks (flutter#36507)

flutter/engine#9935 Fix backspace crash on Chinese devices
flutter#36105 Catch a yaml parse failure during project creation

Use test instead of test_api package in platform_channel_swift exampl…

…e tests (flutter#35942)