* Remove MediaQuery padding for various PopupRoutes that don't extend to the edge of the screen

* bottom sheet doesn't actually need handling. Scaffold does it already. Add test for others.

This fixes DropdownButtons to align their popups correctly in RTL as well.

Also while I was there I fixed the issue with text scale factor in the gallery.

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.

Fix dropdown buttons menus being clipped by the edge of the screen

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

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

* more widget const constructors

* prefer const constructors

* address review comments

* make @immutable const

* fix build

* Manually fix every use of Point.x and Point.y

Some of these were moved to dx/dy, but not all.

* Manually convert uses of the old gradient API

* Remove old reference to Point.

* Mechanical changes

I applied the following at the root of the Flutter repository:

git ls-files -z | xargs -0 sed -i 's/\bPoint[.]origin\b/Offset.zero/g'
git ls-files -z | xargs -0 sed -i 's/\bPoint[.]lerp\b/Offset.lerp/g'
git ls-files -z | xargs -0 sed -i 's/\bnew Point\b/new Offset/g'
git ls-files -z | xargs -0 sed -i 's/\bconst Point\b/const Offset/g'
git ls-files -z | xargs -0 sed -i 's/\bstatic Point /static Offset /g'
git ls-files -z | xargs -0 sed -i 's/\bfinal Point /final Offset /g'
git ls-files -z | xargs -0 sed -i 's/^\( *\)Point /\1Offset /g'
git ls-files -z | xargs -0 sed -i 's/ui[.]Point\b/ui.Offset/g'
git ls-files -z | xargs -0 sed -i 's/(Point\b/(Offset/g'
git ls-files -z | xargs -0 sed -i 's/\([[{,]\) Point\b/\1 Offset/g'
git ls-files -z | xargs -0 sed -i 's/@required Point\b/@required Offset/g'
git ls-files -z | xargs -0 sed -i 's/<Point>/<Offset>/g'
git ls-files -z | xargs -0 sed -i 's/[.]toOffset()//g'
git ls-files -z | xargs -0 sed -i 's/[.]toPoint()//g'
git ls-files -z | xargs -0 sed -i 's/\bshow Point, /show /g'
git ls-files -z | xargs -0 sed -i 's/\bshow Point;/show Offset;/g'

* Mechanical changes - dartdocs

I applied the following at the root of the Flutter repository:

git ls-files -z | xargs -0 sed -i 's/\ba \[Point\]/an [Offset]/g'
git ls-files -z | xargs -0 sed -i 's/\[Point\]/[Offset]/g'

* Further improvements and a test

* Fix minor errors from rebasing...

* Roll engine

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

After this patch, there are three major text input widgets:

 * EditableText. This widget is a low-level editing control that
   interacts with the IME and displays a blinking cursor.

 * TextField. This widget is a Material Design text field, with all the
   bells and whistles. It is highly configurable and can be reduced down
   to a fairly simple control by setting its `decoration` property to
   null.

 * TextFormField. This widget is a FormField that wraps a TextField.

This patch also replaces the InputValue data model for these widgets
with a Listenable TextEditingController, which is much more flexible.

Fixes #7031

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

* address review comments

* use color.shadeXxx instead of color[Xxx]

* remove calls to .shade500 on MaterialColor

* remove calls to .shade200 on MaterialAccentColor

* fix test

Fixes #8235

The latter is the proper spelling.

Fixes #8883

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.

Also use the word "dropdown" consistently over "drop down".

Now that ScrollView supports itemExtent, we can use it in a few more places.

...now that we have generic methods, their types need to be specified too.

Based on P5's experience

When I changed how routes complete their futures, I broke the Dropdown
button because it was still waiting for its own Completer to complete
instead of using the Future returned by push. This patch fixes that
issue.

I've also removed the previous behavior of the DropdownButton forwarding
its text style to its route. The mechansim that we were using doesn't
work properly in all cases. For example, if the DropdownButton is a
child of a LayoutBuilder, then the route will have already built by the
time the DropdownButton gets a chance to forward its text style, causing
an assert in setState.

Finally, I've tweaked PopupMenuButton to work the same way as
DropdownButton in a couple corner cases (e.g., not calling the changed
callback if the button was removed from the tree before the menu
completed its Future).

Fixes #6352

These futures complete when the route is popped off the navigator. This
generalizes and simplifies a mechanism already in place for dialogs and
menus.

Fixes #5283