* Revert "Fix documentation build for window changes. (#21780)"

This reverts commit 931a0468.

* Revert "Migration to PlatformDispatcher and multi-window (#20496)"

This reverts commit 85b0031f.

Extracts a TextRange class with a base and extent, and start(), end(),
collapsed(), and length() getters.

The possibility of reversed base and extent in selections and composing
ranges makes reasoning about them complex and increases the chances of
errors in the code. This change migrates most uses of base and extent in
the text model to start()/end() or position(). The position() method is
intended purely as an aid to readability to indicate that a collapsed
selection is expected at the call site; it also enforces a debug-time
assertion that this is the case.

The space in the argument name was causing this argument to be dropped and the sound and unsound dills to be identical.

* carrying code

* more changes for carrying the code

* rebase changes onto ios-screenshot tests

* adding screenshot capability to text_editing e2e test

* address some comments

* change enable flag for isUnitTestsScreenshotsAvailable

* addressing the reviewer comments

* change the dependency for path

* add to licencense file

* changing goldens commit no. the new commit has the screenshot goldens

* update readme file

* firefox tests needs LUCI changes

* change to release mode since screenshots were taken in release mode

* change window size

* some argument changes

* small comment change

* test the chrome linux tests again

* use roboto font instead of default font

* addressing reviewer comments

* change commit for goldens

* Fix documentation build for window changes.

* Add missing interfaces for web_ui

This reverts commit 5585ed99.

Additionally, the following _flutter.runInView deadlock is fixed.

Previously, a deadlock would occur when service protocol
_flutter.runInView is used to restart the engine wihtout tearing down
the shell: the shared mutex of the service protocol will be locked
during the restart as it's in the middle of handling a service protocol
message; if ServiceProtocol::AddHandler is also called during the
restart, the deadlock happens as AddHandler also requires such lock.

test/integration.shard/background_isolate_test.dart would fail
without this fix.

This is to unblock a Dart -> engine roll.

This is a PR for converting the dart:ui code in the engine to use a multi-window API. The goal here is to convert from the window singleton to an API that has the concept of multiple windows. Also, I'm matching up the new PlatformDispatcher class to talk directly to the PlatformConfiguration class in the engine. I'm not attempting to actually enable creating multiple windows here, just migrate to an API that has a concept of multiple windows. The multi-window API in this PR currently only ever creates one window.

The design doc for this change is here.

The major changes in this PR:

Move the platfom-specific attributes out of Window, and into the new PlatformDispatcher class that holds all of the platform state, so that the platform code need only update the configuration on this class.
Create FlutterView, FlutterWindow, and SingletonFlutterWindow classes to separate out the concepts of a view (of which there may be multiple in a window), a window (of which there may be multiple on a screen, and they host views), and a window where there is only ever expected to be one (this hosts the entire API of the former Window class, and will eventually be the type of the window singleton).
Next step after this PR lands:

Remove the Window class entirely (it is replaced by SingletonFlutterWindow). Some minor changes in the Framework are needed to switch to using SingletonFlutterWindow directly first.

The Window class still exists in this PR, but will be removed as soon as the framework is converted to point to the SingletonFlutterWindow class instead. They share the same API, just have different names (Window is currently a subclass of SingletonFlutterWindow). The intention is that the Window name will be freed up to use as a widget class name in the framework for managing windows. The singleton called window will remain, and keep the same API it has now.

* Move shader.dart into shaders directory
* Add Shader builder basic structure and decls
* rewrite drawVertices with ShaderBuilder
* Fix in parameters in fragment shader to use varying for webgl1

https://skia.googlesource.com/skia.git/+log/e17b0501963a..453f67ff0ade

2020-10-09 ethannicholas@google.com SkSL enum changes
2020-10-09 egdaniel@google.com Suppress vulkan validation layers bug.
2020-10-09 ethannicholas@google.com more SkSL IRNode refactoring
2020-10-09 egdaniel@google.com Make sure the normal GrProgramDesc handles input attachment key.
2020-10-09 michaelludwig@google.com SkDevice::drawSpecial accepts arbitrary matrix v2
2020-10-09 brianosman@google.com Reorganize how SkSL includes are parsed and stored
2020-10-09 johnstiles@google.com Reland "Add sk_Caps.builtinDeterminantSupport and use it in cross()."
2020-10-09 adlai@google.com Reland "Merge GrContext::init into GrDirectContext ..."
2020-10-09 johnstiles@google.com Add test for sk_Caps.mustGuardDivisionEvenAfterExplicitZeroCheck.
2020-10-09 johnstiles@google.com Add test for sk_Caps.inBlendModesFailRandomlyForAllZeroVec.
2020-10-09 johnstiles@google.com Reland "Put top level FPs into their own functions"
2020-10-09 ethannicholas@google.com refactored more SkSL IRNodes
2020-10-09 egdaniel@google.com Reland "Rename GrStencilAttachment class to generic GrAttachment"
2020-10-09 skia-autoroll@skia-public.iam.gserviceaccount.com Roll Chromium from af82ff2606e9 to 4a368eae5a6f (468 revisions)
2020-10-09 skia-autoroll@skia-public.iam.gserviceaccount.com Roll SwiftShader from 5f4e70b81046 to 952149303d06 (1 revision)
2020-10-09 skia-autoroll@skia-public.iam.gserviceaccount.com Roll Dawn from 6b4a4a85dfd7 to 793a07e36636 (12 revisions)
2020-10-09 jvanverth@google.com Revert "Add sk_Caps.builtinDeterminantSupport and use it in cross()."
2020-10-09 brianosman@google.com Remove custom iterators from SkSL::Program
2020-10-08 jvanverth@google.com Revert "Rename GrStencilAttachment class to generic GrAttachment"
2020-10-08 johnstiles@google.com Add sk_Caps.builtinDeterminantSupport and use it in cross().
2020-10-08 michaelludwig@google.com Add SkImageFilters::Shader in place of Paint factory
2020-10-08 michaelludwig@google.com Clip perspective bounds by device clip in SkPDFDevice
2020-10-08 jvanverth@google.com Revert "Merge GrContext::init into GrDirectContext ..."
2020-10-08 jvanverth@google.com Add push constant support to GrCaps.
2020-10-08 adlai@google.com Merge GrContext::init into GrDirectContext ...
2020-10-08 bsalomon@google.com Expose ManagedBackendTexture from BackendTextureImageFactory.
2020-10-08 kjlubick@google.com [canvaskit] Attempt to turn off rtti
2020-10-08 johnstiles@google.com Use SkSTArray to track CFG exits instead of std<set>.

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/skia-flutter-autoroll
Please CC jvanverth@google.com on the revert to ensure that a human
is aware of the problem.

To report a problem with the AutoRoller itself, please file a bug:
https://bugs.chromium.org/p/skia/issues/entry?template=Autoroller+Bug

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

https://dart.googlesource.com/sdk.git/+log/1e7250f91944..712e35f7fd0b

2020-10-09 dart-luci-ci-builder@dart-ci.iam.gserviceaccount.com Version 2.11.0-205.0.dev

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/dart-sdk-flutter-engine
Please CC dart-vm-team@google.com on the revert to ensure that a human
is aware of the problem.

To report a problem with the AutoRoller itself, please file a bug:
https://bugs.chromium.org/p/skia/issues/entry?template=Autoroller+Bug

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