Consolidate duplicated cut/copy/paste/selectall code so it can be done via Actions in the future.

Revert "[EditableText] call `onSelectionChanged` only when there're actual selection/cause changes (#87971)" (#88183)

This reverts commit d056500b.

This reverts commit 31de052e.

This reverts commit e2490f29.

Fixes a bug on Android Gboard where the handle would disappear when dragged within a word.

[TextSelectionControls] move the tap gesture callback into _TextSelectionHandlePainter, remove `_TransparentTapGestureRecognizer`. (#83639)

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.

* Hide selection handles from semantics

* update comment

This reverts commit a46139a2.

A very minimal right-click menu for Mac desktop.

selectionEnabled was not taken into account in:

key movement handling in RenderEditable
mouse drag handling in TextSelectionGestureDetectorBuilder

Exposes the ability to create custom Material text selection toolbar menus, and cleans up the code around these menus.

Fixes a bug where selection handles sometimes showed on web.

* 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>