Merging on read to unbreak the build.

TBR= @Hixie 

Introduce an overall material "color scheme" and to make it possible to redefine how a component's visual properties bind to the color scheme and to the existing material Theme.

Add 'restoreSystemUIOverlays' to SystemChrome to allow simple UI restore after System force changes. (#22221)

* Add 'restoreSystemUIOverlays' to SystemChrome to allow simple UI restore after System force changes.

* use RadioListTile in expansion panels demo

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

* Change file paths to URIs to fix "Could not run configuration in engine" on Windows

Plus unskip test that was failing due to this.

Fixes #21348.

* Remove unused import

* Revert "unnecessary this in field initializers (#22522)"

This reverts commit 5de96bb7.

* Revert "Use unsafePackageSerialization when running tests (#22407)"

This reverts commit 4bd6ad73.

* Revert "Roll engine f6af1f20ba92..b59c86493159 (3 commits) (#22538)"

This reverts commit 85a4591a.

https://github.com/flutter/engine/compare/f6af1f20ba92...b59c86493159

git log f6af1f20ba92..b59c86493159 --no-merges --oneline
b59c86493 Rename the Android fragment support library JAR (flutter/engine#6400)
4213ac19c Add an Android fragment support library to third_party (flutter/engine#6384)
a785b25f4 do not count Hidden nodes at the beginning of the scrollable (flutter/engine#6381)


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

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+/master/autoroll/README.md

If the roll is causing failures, please contact the current sheriff, who should
be CC'd on the roll, and stop the roller if necessary.

Manually roll the engine to the last good commit known to the
auto-roller.

git log 0c2b6dc6e339..f6af1f20ba92 --no-merges --oneline
f6af1f20b Roll src/third_party/skia e8e2bb1384ad..3b8b11e1f912 (4 commits) (flutter/engine#6395)
7f2e2ea4f Roll src/third_party/skia bba2921c1970..e8e2bb1384ad (1 commits) (flutter/engine#6394)
4f69beb27 Roll src/third_party/skia 7f3ceba5dc7f..bba2921c1970 (1 commits) (flutter/engine#6392)
a4a5449d5 Roll src/third_party/skia 22867ce6fe83..7f3ceba5dc7f (1 commits) (flutter/engine#6391)
31278f0b8 Roll src/third_party/skia ebfbf1d749c1..22867ce6fe83 (1 commits) (flutter/engine#6390)
73409c228 Roll src/third_party/skia 93d71666b2f1..ebfbf1d749c1 (1 commits) (flutter/engine#6389)
6f8c827b2 Dart SDK roll for 2018-09-28 (flutter/engine#6388)
6e96ae542 Roll src/third_party/skia cfa315390f04..93d71666b2f1 (6 commits) (flutter/engine#6386)
39dee5b61 Fix Fuchsia build. (flutter/engine#6383)
560eba1a3 Roll src/third_party/skia 87311c6e565f..cfa315390f04 (9 commits) (flutter/engine#6382)

The asset subsystem uses URIs to represent asset locations.  The font manifest
should also use URI-encoded paths instead of unencoded paths taken directly
from the manifest YAML.

Fixes https://github.com/flutter/flutter/issues/19452

* compare to null with ==

* address review comments

This reverts commit b07d986f.

Eliminates the --snapshot and --depfile parameters from the flutter
bundle command. The snapshot parameter is unused in Dart 2 -- code is
built to kernel .dill files and for profile/release builds, then AOT
compiled.

While depfiles are still used in Dart 2 (e.g. by the kernel compiler),
there are enough assumptions in the code that they lie in the default
location (e.g. in the Gradle build) and no reasons to support
user-cusomisation that it makes sense to eliminate the --depfile option
as well, and always use the default location.

This commit also renames 'depFilePath' to 'depfilePath' for consistency
across the codebase.

* Fix internationalization issues with cupertinoTextSelectionControls

Fixes #22247.

* Make Chip press elevation customizable.

* Change flutter tool to not require Fuchsia build args

* Restore code that was commented out.

* Fix style nits.

This also involves switching from Core JIT to App JIT snapshot, and replacing per-isolate VM snapshot with the shared VM snapshot.

For now there is no separate update bundle file, as the generated update gets packaged directly into the APK for testing purposes.

git log 2a22019a8dd8..7dc6b3a8eeef --no-merges --oneline
7dc6b3a8e Skip non-existing libflutters for symbol check (flutter/engine#6374)
0cd9ee372 Add more debug loging to symbol verification script (flutter/engine#6372)
fcb3c519b Roll src/third_party/skia b726c89ff8a8..d004e0b55c4a (1 commits) (flutter/engine#6373)
5e3d786d0 Use nm from buildtools (flutter/engine#6371)

For retained rendering, we don't want to push the offset down to each leaf layer. Otherwise, changing an offset layer on the very high level could cascade the change to too many leaves, which means that we can't retain them.

To not push the offset downwards, we simply push a TransformLayer when there's an offset. Skia has a fast path for concatenating scale/translation-only matrix so this operation should be fast (no performance regression is measured on Moto G4).

This is our first step towards https://github.com/flutter/flutter/issues/21756

Through some integration testing checks, it looks like it's possible
that connecting to a new instance of the Dart VM may fail even after the
Dart VM instance has been discovered.

This means that _getDartVm should be checked in more than just one
place.  This changes the function to return a null value in the event of
a 'well-known' exception (`HttpException` or `TimeoutException`
specifically).

As a result, code now calling _getDartVm checks for null and handles it
as appropriate, and the DartVm RPC calls are all updated with
consistent timeout params.

git log 2a22019a8dd8..9ae1c14c3515 --no-merges --oneline
9ae1c14c3 Update observatory-port default behavior description. (flutter/engine#6365)
5550fdbc2 Remove cull_rects_ from SceneBuilder (flutter/engine#6352)
75e404f39 Roll src/third_party/skia 656cefe65d62..b726c89ff8a8 (8 commits) (flutter/engine#6370)

See https://github.com/flutter/flutter/pull/22278#pullrequestreview-159230244

This reverts commit 7b2d3462.