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 adds a slider demo with a custom theme to the gallery.

In the process of adding this, I decided to add a text theme to the SliderThemeData, since it's a pain to change the text style on the value indicator otherwise.

This implements an update to the look of the Slider widget.

Specifically, it does the following:

* Adds the ability to customize the colors of all components of the slider
* Adds the ability to customize the shape of the slider thumb and value indicator
* Adds the ability to show the value indicator on continuous sliders
* Updates the default value indicator to be a "paddle" shape with new animations.
* Changes the tick marks to be visible all the time on discrete sliders
* Fixes a memory leak of an animation controller.
* Removes "thumbOpenAtMin" flag, which is no longer needed, and can be emulated by the
custom thumb shape support. It was not widely used.
* Adds tests for all of the new features.

Previously it was (arbitrarily?) set to 144.0 pixels wide.

* more widget const constructors

* prefer const constructors

* address review comments

Aggressively apply the const lint.

* Add cupertino to gallery and add CupertinoButto

* Use single quotes

* Add disabled state

* Some review notes

* Make button animation more responsive and tweak timing

* Renamed things Cupertino

* Button with background, move cupertino demos, move material demos

* Move 2 level list too

* Refactor various demo route names

* Some review notes

* More reviews and add test

* Linter as

* Move private constant up

This patch changes the default appearance of Slider to not have the
thumb be an open circle at its minimum position. The `thumbOpenAtMin`
property can enable drawing an open thumb at the min position, which was
the previous behavior.

Fixes #6941

Previously, it was unclear that the middle slider was disabled.

Fixes #5517

We now use the `@required` annotation to encourage developers to
explicitly set onPressed and onChanged callbacks to null when that would
disable the widget.

Fixes #287

Address feedback from material design team.

Fixes #4144
Fixes #4145
Fixes #4146

Use the route table to generate the list of screens to test in the smoke test.

Fixes #1541

* justifyContent -> mainAxisAlignment
* alignItems -> crossAxisAlignment
* FlexJustifyContent -> MainAxisAlignment
* FlexAlignItems -> CrossAxisAlignment

Fixes #231

* left -> leading (Removes an LTR bias)
* center -> title (Widget was actually centered)
* right -> actions (Removes an LTR bias, asymmetric with leading)

Fixes #2348

This patch renames StatelessComponent to StatelessWidget and
StatefulComponent to StatefulWidget.

Fixes #2308

Fixes #1382

Previously we used a positional argument for widgets that had multiple
children. Now we use a named argument that defaults to an empty list.

Fixes #241

Have the widget support min/max arguments to make the widget easier to
use. Also, fix the dragging so it's actually where the knob is.

Also, clean up the main.dart file to prepare for making dynamic theme changes.

Also, have the slider draw different in its disabled state.

The slider in this patch works, but it's missing animations and many subtle
touches.