* 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>

* Ignore unmounted parent when _debugVerifyGlobalKeyReservation (#62055)

* Ignore unmounted parent when _debugVerifyGlobalKeyReservation (#62055)

* Ignore unmounted parent when _debugVerifyGlobalKeyReservation (#62055)

* Ignore unmounted parent when _debugVerifyGlobalKeyReservation (#62055)

* Ignore unmounted parent when _debugVerifyGlobalKeyReservation (#62055)

* Ignore unmounted parent when _debugVerifyGlobalKeyReservation (#62055)

* Revert "Revert "Add clipBehavior to widgets with clipRect (#55977)" (#58344)"

This reverts commit 1d395c5e.

* Add missed Overflow

* add language version 2.8 in packages/flutter

* enable non-nullable analyzer flag

This reverts commit cd593dae.

* Add clipBehavior to RenderFlex

* Add clipBehavior to FittedBox

* Add clipBehavior to Flex and FittedBox

* Add clipBehavior to UnconstrainedBox

* Add clipBehavior to Stack and Wrap

* Add clipBehavior to TextEditable

* Add clipBehavior to ListWheelScrollView

* Add clipBehavior to SingleChildScrollView

* Add clipBehavior to RenderViewportBase's widgets

Those widgets are NestedScrollView and ShrinkWrappingViewport.

* Fix tests

* Remove enum Overflow and fix typo

* Remove clipToSize

* Analyze fix

* Remove Mixin and other small fixes

* Fix tests and respect Stack's default clipBehavior

* Add Overflow back to make it non-breaking

* Restore clipBehavior to make it non-breaking

* Small fixes

* Fix rebase

* Reland "Fix custom Elements that wants to decorate State.build (#49925)" (#50068)"

This reverts commit 62fdcb03.

Updates stack filtering for new frames in stateful element

This reverts commit a1143d17.

* reland "Refactors global key duplication detection (#46183)"

This reverts commit d2b66dbf.

* fix test

This reverts commit 340b193f.

* add key to constructors of public widgets

* fix a test

* fix a test

* 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

* Add structured errors in Animations, TabView, ChangeNotifier

* Add structured error on MaterialPageRoute, BoxBorder, DecorationImagePainter, TextSpan

* Add structured errors in Debug

* Fix test errors

* Add structured errors in Scaffold and Stepper

* Add structured errors in part of Rendering Layer

* Fix failing test due to FloatingPoint precision

* Fix failing tests due to precision error and not using final

* Fix failing test due to floating precision error with RegEx instead

* Add structured error in CustomLayout and increase test coverage

* Add structured error & its test in ListBody

* Add structured error in ProxyBox and increase test coverage

* Add structured error message in Viewport

* Fix styles and add more assertions on ErrorHint and DiagnosticProperty

* Add structured error in scheduler/binding and scheduler/ticker
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

* Add structured error in AssetBundle and TextInput
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

* Add structured errors in several widgets #1
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

* Remove unused import
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

* Add assertions on hint messages
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

* Fix catch spacing
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

* Add structured error in several widgets part 2 and increase code coverage
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

* Add structured error in flutter_test/widget_tester

* Fix floating precision accuracy by using RegExp
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

* Remove todo to add tests in Scaffold showBottomSheet
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

* Fix reviews by indenting lines and fixing the assertion orders
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

* Fix failing tests due to renaming class
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

* Try skipping the NetworkBundleTest
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

* Remove leading space in material/debug error hint
Signed-off-by: Albertus Angga Raharja <albertusangga@google.com>

* add missing trailing commas

* add more missing trailing commas

* add more and more missing trailing commas

Breaking change to extremely rarely used ParentDataWidget.debugDescribeInvalidAncestorChain api changing the return type of the method from String to DiagnosticsNode.

This reverts commit 77ab0b83.

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

It was causing problems rolling Flutter into Fuchsia

* re-enable lint unnecessary_const

* remove trailling whitespaces

* remove unnecessary const (after merge)

This reverts commit cc1cf13e.

This allows const GlobalObjectKey(0) to be concisely formatted as
[GlobalObjectKey int#0000] in both Dart 2 and Dart 1 modes.

Without this change it would be formatted as
[GlobalObjectKey<State<StatefulWidget>> int#0000] because in Dart 2
types are instantiated to bounds.

In addition to retaining general readability this also fixes few
tests that rely on this short string representation (see
test/widgets/global_keys_duplicated_test.dart).

Notice that the default alignment for Stack needs to know the
directionality.

In some cases, the element tree is not clean but we are required to dump the tree anyway.
To avoid crashing in those cases, we return an explicit null node.