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

This reverts commit 931a0468.

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

This reverts commit 85b0031f.

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.

for Android and iOS and remove offset from scenario_app test for iOS and Android.

Add copyright headers in a few files where they were missing.

Trim trailing blank comment line where present, for consistency with
other engine code.

Use the standard libtxt copyright header in one file where it differed
(extra (C) and comma compared to other files in libtxt).

This also amends tools/const_finder/test/const_finder_test.dart to look
for a const an additional four lines down to account for the copyright
header added to the test fixture.

This reverts commit 8d08e6c3.

* Initialize locale from FlutterEngine

git log 1402e8e1a42d..e21c4353006d665e7c4378413ad4d237f18437e0 --date=short --first-parent --format='%ad %ae %s'
2020-03-30 jsimmons@google.com [dart:io] Fuchsia version of SocketBase::AvailableDatagram
2020-03-30 jwren@google.com Fix the dart help test output by implementing printUsage() in the test command
2020-03-30 sstrickl@google.com [vm/aot] Drop some Function objects not needed at runtime.
2020-03-30 mit@google.com Roll pub to latest version
2020-03-30 scheglov@google.com Document differences in inheritance tests between CFE and analyzer.
2020-03-30 scheglov@google.com Revert "Fix the result type of binary expression with the left TypeParameter extends num type."
2020-03-30 johnniwinther@google.com [cfe] Handle member signatures from dill in class hierarchy builder
2020-03-30 eernst@google.com [cfe] Add missing .next in endImplicitCreationExpression
2020-03-30 jensj@google.com [CFE] Add api to get language version for uri
2020-03-30 johnniwinther@google.com [cfe] Don't include member signature in id testing by default.
2020-03-30 johnniwinther@google.com [kernel] Make verbose qualified names conditional in toStringInternal
2020-03-30 devoncarew@google.com [dartdev] test the dartdev analyze --fatal-warnings option
2020-03-30 brianwilkerson@google.com Enable omit_local_variable_types in analyzer_plugin
2020-03-30 brianwilkerson@google.com Enable omit_loval_variable_types in analysis_server_client
2020-03-30 brianwilkerson@google.com Enable omit_local_variable_types in analysis_server
2020-03-30 keertip@google.com Add guard code for exceptions from ResourceProvider when reading contents.
2020-03-29 scheglov@google.com Fix the result type of binary expression with the left TypeParameter extends num type.
2020-03-28 scheglov@google.com Fix for creating a setter when the target is in a part without a library.
2020-03-28 brianwilkerson@google.com Remove the need for a --corpus flag from completion_metrics
2020-03-28 brianwilkerson@google.com Remove the need for a --corpus flag from relevance_metrics
2020-03-28 brianwilkerson@google.com Remove an unused variable in AstBuilder
2020-03-28 brianwilkerson@google.com Sort non-generated files in analyis_server and add a test to ensure they stay that way
2020-03-28 scheglov@google.com Add LibraryElement.languageVersionMajor/Minor
2020-03-28 scheglov@google.com Report exceptions with the file content for getResult().
2020-03-28 scheglov@google.com Fix crash in index when the invoked element is multiply defined.
2020-03-28 eernst@google.com Add implementation of several parts of non-function type aliases
2020-03-28 johnniwinther@google.com [cfe] Groups standard tests in update_expectations.dart
2020-03-28 sra@google.com [dart2js] Specialize is- and as- tests for nullable types
2020-03-28 rmacnak@google.com [vm] Do not allocate when evaluating arguments to StorePointer.
2020-03-28 rmacnak@google.com [vm] Fix some data races with profiler samples.
2020-03-28 alexmarkov@google.com Revert "[analyzer] Remove deprecated language version API"
2020-03-27 nshahan@google.com [ddc] Cleanup cast in dart:_runtime lib
2020-03-27 brianwilkerson@google.com New relevance values for UriContributor
2020-03-27 brianwilkerson@google.com New relevance values for KeywordContributor
2020-03-27 brianwilkerson@google.com New relevance values for VariableNameContributor
2020-03-27 paulberry@google.com Migration: don't use union edges for field formal parameters
2020-03-27 rmacnak@google.com [vm] Fix memory leak in profiler when some samples are in native functions without a dynamic symbol.
2020-03-27 bkonyi@google.com [ package:dds ] Fixed smoke_test.dart which was unable to launch smoke.dart due to use of a relative path.
2020-03-27 zichangguo@google.com [dart:io] Fix parsing the response of the HTTP connect method
2020-03-27 mfairhurst@google.com [nnbd_migration] Add confirm dialog to "apply migration" button
2020-03-27 rmacnak@google.com [test] Update use of Null as a bottom type to Never.
2020-03-27 eernst@google.com Added new tests.
2020-03-27 alexmarkov@google.com [vm/nnbd/aot/tfa] Support NNBD strong mode in type flow analysis
2020-03-27 mfairhurst@google.com [analyzer] Remove deprecated language version API
2020-03-27 brianwilkerson@google.com Update relevance computations in LocalReferenceContributor
2020-03-27 brianwilkerson@google.com Update relevance values for LocalLibraryContributor and ImportedReferenceContributor
2020-03-27 sigurdm@google.com Bump pub version
2020-03-27 sigmund@google.com [dart2js] Update test-matrix to specify the libraries.json to dart2js batch runner
2020-03-27 jcollins@google.com Update dartdoc to 0.30.3.
2020-03-27 alexmarkov@google.com [vm/nnbd] Fix marking static fields as late
2020-03-27 scheglov@google.com Fix for completion in formal parameters of GenericFunctionType.
2020-03-27 mfairhurst@google.com [nnbd_migration] Add a "rerun from sources" button.
2020-03-27 mfairhurst@google.com Revert "[analyzer] Copy result of allSupertypes for now."
2020-03-27 scheglov@google.com Merge types during instance member inference.
2020-03-27 scheglov@google.com Catch linking exceptions and include the cycle file contents.
2020-03-27 johnniwinther@google.com [cfe] Handle throw in return type inference
2020-03-27 alexmarkov@google.com [vm/aot/tfa] Fix tracking of parameters in async_op closures
2020-03-27 zichangguo@google.com Reland "[dart:io] Fix hanging on zero-length datagram"
2020-03-27 athom@google.com [infra] Update checked-in SDKs to 2.8.0-dev.18.0
2020-03-27 johnniwinther@google.com [cfe] Add tool for updating all id-tests and expectation tests
2020-03-27 athom@google.com [homebrew] Add support for the beta channel formula
2020-03-27 brianwilkerson@google.com Sort files in analysis_server_client and analyzer_plugin and add tests to keep them that way
2020-03-27 askesc@google.com [benchmark] Benchmark for UTF-8 decoding with typical data.
2020-03-27 jensj@google.com [parser] Change recovery for constructor; report errors in bad constructors
2020-03-27 johnniwinther@google.com [cfe] Use legacy erasure before argument inference
2020-03-27 jensj@google.com [kernel] Polish toString implementation on AST nodes
2020-03-27 sra@google.com [dart2js] Print non-minified init object as multiline
2020-03-27 karlklose@google.com [infra] Handle test steps with no tested configuration
2020-03-27 zichangguo@google.com [dart:io] remove available at _NativeSocket::Read
2020-03-27 nbosch@google.com Remove a stale TODO comment
2020-03-27 rnystrom@google.com Add other breaking changes to CHANGELOG.
2020-03-27 liama@google.com [nnbd/test] Fix tests that are failing in strong mode
2020-03-26 rnystrom@google.com Fix Markdown link in CHANGELOG and remove duplicate entry.
2020-03-26 brianwilkerson@google.com Compute new relevance in LocalConstructorContributor
2020-03-26 kevmoo@google.com DEPS: remove references to pkg:package_resolver
2020-03-26 jcollins@google.com Add a way for analysis_server to open communication via a IsolateChannel.
2020-03-26 scheglov@google.com Use NNBD_TOP_MERGE to combine signatures of the inherited members.
2020-03-26 nshahan@google.com [ddc] Update types in JSArray operations
2020-03-26 srawlins@google.com Migrator: Add an auth token for the preview site
2020-03-26 dacoharkes@google.com [vm] Add Reachability Fence Instruction
2020-03-26 sra@google.com [benchmark] Make BigIntPrintParse be NNBD-agnostic
2020-03-26 rnystrom@google.com Bump the SDK constraint on dart_internal to permit Dart 2.8.0.
2020-03-26 kevmoo@google.com Update changelog with dart2js --package-root removal
2020-03-26 alexmarkov@google.com [vm/corelib] Specify types explicitly in convert_patch.dart
2020-03-26 scheglov@google.com Update checkForWrongTypeParameterVarianceInMethod(), fix NPE.
2020-03-26 mfairhurst@google.com [analyzer] Copy result of allSupertypes for now.
2020-03-26 scheglov@google.com Add  more data to debug crash in buildExportNamespace().
2020-03-26 scheglov@google.com Fix for crash when FieldFormalParameter in a local function has annotations.
2020-03-26 bkonyi@google.com [ VM / DartDev ] Disable DartDev for the 2.8 release
2020-03-26 kevmoo@google.com Remove unused package-root option and configuration from dart2js
2020-03-26 srujzs@google.com [dart:html] Add nullability to EventTarget conversion
2020-03-26 kevmoo@google.com DEPS: Bring in latest pub
2020-03-26 dacoharkes@google.com [vm/ffi] Mark Struct as entrypoint
2020-03-26 jensj@google.com [analyzer] Sort entries in error.dart and syntactic_errors.dart
2020-03-26 karlklose@google.com [infra] pkg/smith: custom test runner steps are test steps

This reverts commit 2627634b.

* opt out dart:ui from nnbd

* Annotate tests as well

This reverts commit f456423c.

This is being reverted because it caused flutter/flutter#45098
(images don't load on iOS).

This reverts commit 2dcfaaeb.

This reverts commit 618e6666.

Revert "Always use `IOSGLContextSwitch` to access EAGLContexts to prevent plugins from polluting Flutter's EAGLContext (#13314)" (#13753)

Always use `IOSGLContextSwitch` to access EAGLContexts to prevent plugins from polluting Flutter's EAGLContext (#13314)

When gr context is changed (this happens when sending the app to the background and then to the foreground) we need to update it for all the platform view overlay surfaces.

The update logic was caching the previous gr context to figure if it had to be updated, but after updating it for a given overlay we were updating the cached context. In apps with multiple platform views this will result in overlays with a stale gr context.

This fixes flutter/flutter#36437
And I believe it should fix flutter/flutter#36999 as well (though I don't have repro code to verify).