* Introduce a Directionality inherited widget which sets the ambient LTR vs RTL mode (defaulting to null, which means you cannot use directionality-influenced values).

* Make it possible to configure Padding (including Container.padding and Container.margin) using a directionality-agnostic EdgeInsets variant.

* Provide textDirection and verticalDirection controls on Row and Column to make them RTL-aware.

* Introduce a variant of FractionalOffset based on the EdgeInsets variant. Not yet actually used.

* Fix all the tests that depended on Row defaulting to LTR.

when describing non-nullable method arguments.

* Allow tapping on TabBar indicator to switch tabs

* fix semantics

* review comments

Add Diagnosticable base class and documentation

Refactor RenderObject.toStringDeep and Widget.toStringDeep code to use DiagnosticsNode.

* [accessibility] Announce index if tab in tab bar

* added TODO

* Add SemanticsNode.isSelected flag

* Adds example usage to TabBar

See also https://github.com/flutter/engine/pull/3764

* Review comments

* whitespace fixes

* Fix doc ref and update engine roll

Moved everything icons related except the material icon button and the materialicon font list to widget

This patch finishes documenting all the symbols except for those related to
scrolling.

backgroundColor -> color
backgroundImage -> image
BackgroundImage -> DecorationImage

This patch reworks some of the guts of scrolling to make it easier to
implement nested scrolling effects. The actually nested scrolling effect
will be included in a later patch.

* more widget const constructors

* prefer const constructors

* address review comments

* make @immutable const

* fix build

* Scaffold appBar is-a PreferredSizeWidget, etc

* Updated

* Updated per review feedback

I can't figure out if this is genius or a giant hack.

This lets you use DefaultTextStyle.merge and IconTheme.merge without
specifying a BuildContext. It automatically merges in at the
appropriate place in the tree using a Builder widget.

Rename State.config to State.widget
Rename State.didUpdateConfig to State.didUpdateWidget
Renamed all State subclasses' local variables named config to something else

Add some commentary around DefaultTabController.initialIndex.
Fix typo (nuull) in TabBar.
Make some asserts more detailed.

* add @required when there's an assert not null

* address review comments

Fixes issue with removing and re-adding a toolbar which is connected to the same TabBarController (#9015)

The new name matches the style guide. (The old name was just old and predated
the style guide.)

Fixes #8000

Covers lib/ in package:flutter.

* Adds un/selected text styles to the TabBar widget

* Adding documentation on fallback values. Modified fallback on unselectedLabelStyle.

Previously we called onPageChanged when the scroll ended, but that is too late.
Now we call onPageChanged when we cross the halfway mark, which, for example,
makes the tab indicator update earlier.

Fixes #8265

Rename all the "2" classes related to scrolling to remove the "2". Now that the
old scrolling code is gone, we don't need to use the suffix.

Fixes https://github.com/flutter/flutter/issues/8017

All clients have moved to PageView.

Tabs are now fully driven by slivers.