Adds `Scaffold#endDrawer` property to supply a second drawer to a Scaffold.

Unlike FractionalOffset, Alignment uses the center as the zero of the
coordinate system, which makes the RTL math work out much cleaner.

Also, make FractionalOffset into a subclass of Alignment so that clients
can continue to use FractionalOffset.

* SafeArea

* AnimatedSafeArea

* AppBar test

* Apply feedback

* Allow apps to change the title margin in app bar.

* Fix documentation links and the shouldLayout function in the _ToolbarLayout

* Rename margin to spacing

* review comments

* Test fixes. Now we also test the width to make sure enough space is left for trailing widget

* Expose the middle spacing default and use it in app bar.

* Fix analyzer break

* Doc fixes due to review

This reverts commit da0602fe.

Fixes #11381

* Support automaticallyImplyLeading param in AppBar

* Review fixes

* fix review comments

* Revert "Revert "Change all ocurrences of '$runtimeType#$hashCode' to use the idAndType method. (#10871)" (#10880)"

This reverts commit ceb814aa.

Revert "Change all ocurrences of '$runtimeType#$hashCode' to use the idAndType method. (#10871)" (#10880)

This reverts commit d46e208b.

* Change all instances of '$runtimeType#$hashCode' to use the describeIdentity method.

The describeIdentity method generates a shorter description with a consistent length
consisting of the runtime type and the a 5 hex character long truncated
version of the hash code.

Extract layout logic in material app bar to a common file that can be reused for cupertino

Minor fixes throughout, e.g. removing trailing commas from the end of sample code expressions, changing headings to "sample code" more consistently, removing stale todos, fix typos in a private method name, minor grammar fixes, added some clarifications to CircularProgressIndicator, LinearProgressIndicator, CrossAxisAlignment, added some cross-references to various members, made it slightly clearer that layout algorithms are implementation details.

Clarified "elevation" throughout.

Added docs to InkResponse and InkWell.

Added sample code for: SliverAppBar, Card, ListTile, EdgeInsets, Row, Column, CustomScrollView, ListView, SliverFixedExtentList, and SliverGrid.

Fixes #10317.
Fixes #10316.
Fixes #10267. (sort of, see comment therein)
Fixes #9331. (sort of, see comment therein)
Fixes #9407. (sort of, see comment therein)

I added some tests for the bug that I fixed.
I added docs for IconButton and AppBar.
I added some new constructors for FractionalOffset.

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

Also, I had a question about flutter_test matchers and our style guide
says that when I have a question I should update the docs so I did
that and then got a bit carried away.

Fixes https://github.com/flutter/flutter/issues/9342

If there is more than one action, then the AppBar should align the title to the
left according to
<https://material.io/guidelines/platforms/platform-adaptation.html#platform-adaptation-platform-recommendations>

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

* 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

* Wiring fullscreen dialog

* Make fullscreen dialog use the new API. Add tests

* Review notes

* Move field back up

* final

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

Fixes #8000

Changing to an inline function, making the type more clear.

* prefer const constructors

* fix double throw

Covers lib/ in package:flutter.

This widget is useful on its own. This patch factors it out of AppBar so
folks can use it separately.

Fixes #8489