Applies #78522 to existing code, by replacing all occurrences of LogicalKeySet to SingleActivator or ShortcutActivator.

Update samples to use repo analysis options, Fix sample templates and a ton of analyzer issues (#77868)

Text editing shortcuts involving the arrow keys are no longer handled by RenderEditable's RawKeyboardListener, they use the new Shortcuts setup.  First PR in a plan to port all text editing keyboard handling to shortcuts.

This reverts usages of the binding's platformDispatcher to use window again temporarily, because there isn't a TestPlatformDispatcher yet, and so some tests were failing because they mocked the TestWindow to return certain things (locales) that were returning the real values instead of the test values.

Once I've created a TestPlatformDispatcher to allow fake data to be passed to it, we can go back to using the platformDispatcher in all of these places

Remove references to `Window`, and switch usages to `PlatformDispatcher` or `SingletonFlutterWindow` (#69617)

* Remove references to dart:ui.Window, and point usages to PlatformDispatcher or SingletonFlutterWindow, as appropriate

* remove new test platform dispatchers

* Amend documentation

- - When I added notification of key events before processing them as text, it made it so that shortcut key bindings like the spacebar would prevent spaces from being inserted into text fields, which is obviously not desirable (and so that change was reverted). At the same time, we do want to make it possible to override key events so that they can do things like intercept a tab key or arrow keys that change the focus.

This PR changes the behavior of the Shortcuts widget so that if it has a shortcut defined, but no action is bound to the intent, then instead of responding that the key is "handled", it responds as if nothing handled it. This allows the engine to continue to process the key as text entry.

This PR includes:

- Modification of the callback type for key handlers to return a KeyEventResult instead of a bool, so that we can return more information (i.e. the extra state of "stop propagation").
- Modification of the ActionDispatcher.invokeAction contract to require that Action.isEnabled return true before calling it. It will now assert if the action isn't enabled when invokeAction is called. This is to allow optimization of the number of calls to isEnabled, since the shortcuts widget now wants to know if the action was enabled before deciding to either handle the key or to return ignored.
- Modification to ShortcutManager.handleKeypress to return KeyEventResult.ignored for keys which don't have an enabled action associated with them.
- Adds an attribute to DoNothingAction that allows it to mark a key as not handled, even though it does have an action associated with it. This will allow disabling of a shortcut for a subtree.

* migrate material to nullsafety

* address review comments

* address review comments

* fix build

* 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>

A frequent request from the last Flutter developer survey was for an easier method of testing light/dark mode changes. Currently, a user needs to manually change the theme settings or adjust phone settings to see the difference. Instead we should add a toggle from the CLI, and eventually devtools/Intellij/Vscode that allows developers to override the current setting.

Fixes #59495

Adds flutter.ext.brightnessOverride service protocol which either queries the current platform brightness, or overrides it to a new value. This accepts either Brightness.light or Brightness.dark as a value.

Adds a CLI toggle b which allows the setting to be toggled manually.

Requires an update to the MediaQuery, to conditionally use a debug override when not in release mode

* add language version 2.8 in packages/flutter

* enable non-nullable analyzer flag

This updates the Action API in accordance with the design doc for the changes: flutter.dev/go/actions-and-shortcuts-design-revision

Fixes #53276

This PR adds the linux and windows target platform enum values, along with automatically setting the defaultTargetPlatform to the appropriate value on those platforms.

Fixes #31366

* Fix doc typo

This PR is a follow up of https://github.com/flutter/flutter/pull/42640
Some changes of that PR includes redundant changes using FlutterError.fromParts constructor even though it's not necessary.

Some minor changes are:

- Remove one unnecessary todo
- Fix indent consistencies