* Reland "Nnbd widgets (#64672)"

This reverts commit 127e6790.

* remove non-nullable enforcement on AsyncSnapshot

* make data param non-nullable for AsyncSnapshot.withData

* make some Text params non-nullable

This reverts commit e682ec71.

* migrate widget to nullsafety

* remove double blank line after license

* address review comments in actions.dart

* nullable ObjectKey.value

* use local variable oldElement

* make State.build non-nullable

* make State.context non-nullable

* newline at eof

* make ProxyWidget.child non-nullable

* make _InactiveElements.debugContains non-nullable

* make Element.depth non-nullable

* make ProxyElement.build non-nullable

* make StatefulElement.state non-nullable

* remove 'Notice that'

* avoid cast of list in RenderObjectElement.updateChildren

* make IndexedSlot.value non-nullable

* avoid cast of list in MultiChildRenderObjectElement.mount

* make some WidgetsApp parameters non-nullable

* hitTest take non-nullable position

* make ScrollableState.position non-nullable

* use _pixels instead of pixels

* make ViewportOffset.pixels non-nullable

* make param and return type of IndexedWidgetBuilder non-nullable

* unused_import

* make context param non-nullable for Builder in animated_list.dart

* make ScrollMetrics.viewportDimension non-nullable

* make ScrollMetrics.{min,max}ScrollExtent non-nullable

* make _Location.file non-nullable

* _WidgetForTypeTests.createElement throw UnimplementedError

* update _NullWidget.build error message

* make _ShortcutsState.manager non-nullable

* Fix childCount issues for NNBD

* fix childCount computation on web

* increase max value on js side to compute childCount

* make aspect parameter of dependOnInheritedWidgetOfExactType nullable

* merge has{min,max}ScrollExtent into hasScrollExtents

* update focus_manager.dart

* address review comments in icon.dart

* address review comments in image.dart

* address review comments in routes.dart

* address review comments in scroll_activity.dart

* update doc comments

* make UserScrollNotification.direction non-nullable and required

* rename hasScrollExtents to hasContentDimensions

* unnecessary late
Co-authored-by: Ian Hickson <ian@hixie.ch>

Have AndroidViewController extend PlatformViewController and add support for hybrid platform views (#60320)

* add language version 2.8 in packages/flutter

* enable non-nullable analyzer flag

* add key to constructors of public widgets

* fix a test

* fix a test

This normalizes all of the "See also" blocks in comments so that they conform in style.

They all look like this now:

  /// See also:
  ///
  ///  * [MyFavoriteWidget], because it's cool.
  class Foo {}

I removed some useless "See also" blocks, and added commentary to ones that were just "bare" references.

This is my penance for adding so many bullets that are "-" instead of "*". :-)

* Update project.pbxproj files to say Flutter rather than Chromium

Also, the templates now have an empty organization so that we don't cause people to give their apps a Flutter copyright.

* Update the copyright notice checker to require a standard notice on all files

* Update copyrights on Dart files. (This was a mechanical commit.)

* Fix weird license headers on Dart files that deviate from our conventions; relicense Shrine.

Some were already marked "The Flutter Authors", not clear why. Their
dates have been normalized. Some were missing the blank line after the
license. Some were randomly different in trivial ways for no apparent
reason (e.g. missing the trailing period).

* Clean up the copyrights in non-Dart files. (Manual edits.)

Also, make sure templates don't have copyrights.

* Fix some more ORGANIZATIONNAMEs

Currently the surface of a platform view is only created only one when the state of PlatformViewLink is created. When the PlatformViewLink widget changes, the PlatformViewController in the corresponding state is also updated. Just the surface is not updated even though it depends on the controller.

This PR changes this behavior to recreate the surface whenever the controller is updated.

* Implement HtmlView, the platform view widget for Flutter Web

* Document HtmlView constructor

* Respond to review comments

* - Rename to HtmlElementView
- Assert running on web
- Move test to Chrome-only file

* Add viewType to PlatformViewLink

Re-initialize the controller when the viewType changes.

* Re-word `PlatformViewLink` doc comment for clarity

In the build of PlatformViewLink, Added a FocusNode wrapping around the surface widget.
The focus node will ask platform view to clear its focus when necessary through [PlatformViewController.clearFocus].
The platform view can notify the framework it wants to gain focus by calling [PlatformViewCreationParams.onFocusChanged]

PlatformViewLink, handling creation of the PlatformViewSurface and dispose PlatformViewController (#37703)

* link

* review fixes

* review fixes

* remove extra line

* painting and semantics

* more comments

* fixing ci

* review fixes

* add assert for id

* rename custom layer factory to layer builder

* review updates

* partial review fixes

* some doc updates

* more doc updates

* only expose getter for id in PlatformViewController

* doc updates/removing all the  references

* remove extra

* more doc updates

* some doc updates

* more doc fixes

* review fixes

When an AndroidView gains focus we invoke the(newly introduced)
'TextInput.setPlatformViewClient' text_input system channel method
which sets the platform view as the text input target.

When the AndroidView loses focus we send a clearFocus message to
platform views system channel(so the engine can clear the focus from
the platform view).

This PR is going to land before the engine implementation is rolled to
the framework, we swallow MissingPluginException for the newly
introduced method channel methods so this is a no-op before the engine
is ready(after the engine is rolled with the corresponding change I'll
remove the logic to swallow the exceptions).

The engine counterpart is in: flutter/engine#9203

Corects a bnuch of typeos throuhgout teh Fluter codebsae.

Made use of the `misspell` tool:
https://github.com/client9/misspell

The PlatformViewsService listens for `viewFocused` calls on the
platform_views system channel and fires a callback that focuses the
focus node for the relevant AndroidView widget.

It looks like the documentation for UiKitView has been copy-pasted from AndroidView or so.
This PR fixes the word 'Android' to 'iOS' for the `creationParams`.

Adds a note mentioning that this is still a release preview.

Re-landing #23412 with a fix to the PlatformLayer's addToScene signature.

#23412 was broken by the change to Layer done in #23434.

It seemed green as the presubmits were done before #23434 was landed, and when #23412 landed it broke the build.

Reverts #23779

This reverts commit 67ffe1c2.

This PR adds the full framework stack (layer->render object->widget, and
service) for embedding iOS views with minimal functionality.

I allowed myself to throw the entire framework stack in one PR since we're mostly
mirroring the structure we already established for embedded Android views, so this PR
is a little bigger than usual. I'm happy to break it down to the
different pieces of the stack if reviewers prefer.

Specifically this PR adds:
* A UiKitView widget for embedding a UIView in the widget tree.
* A RenderUiKitView which is the render object for showing a
  UIView.
* A PlatformViewLayer which denotes the position of a UIView in the
  layer tree.
* The iOS platform_views system channel client code in services/platform_views.dart
* Splits the fake platform views controller to an Android and iOS
  controllers.

TBD in following PRs:
* Plumb the layout direction all the way to the platform view (currently
  there is still no engine support for it).
* Integrate with gesture arenas (engine support is still missing as
  well).

Before this PR AndroidView's gestureRecognizers field was a list of
gesture recognizers. This was problematic as when the widget was rebuilt
with the same gesture recognizer instances we would try to re-join the
recognizers to a gesture arena team and crash (as a OneSeqeunceGestureRecognizer
team can only be set once).

With this change, we instead take a set of factories.
This allows AndroidView to create the gesture recognizers just before
adding them to the team, and thus be sure that they are only added once to a
team.

The factories are identified by the type of the object they create, this
allows AndroidView to know when it is given an equivalent set of gesture
recognizer factories, and do nothing in that case.