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.

* Built first tab

* Small additions

* started tab 3

* Need color arithmetics

* tab 2 built

* finalize

* lint and tests

* review

* Reapply docs after rebase

* use color.computeLuminance

* linter

* nit

This lets us preview widgets in the gallery using small, normal, large, and HUGE text.

Added selections to the main drawer for these options. Defaults to "normal", obviously.

See https://github.com/flutter/flutter/issues/9473

* make @immutable const

* fix build

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

The stocks routing table is a clearer example of how to do this.
People know to look at the gallery, but don't think to look at the
stocks version. This points them from the former to the latter.

* use color.shadeXxx instead of color[Xxx]

* remove calls to .shade500 on MaterialColor

* remove calls to .shade200 on MaterialAccentColor

* fix test

Fixes https://github.com/flutter/flutter/issues/5066

Add update dialog to gallery, open source part of #4626

This patch is a warmup to improving the visuals in the calculator demo.

Related to #4535

Fixes 3544

Turns out we have the same demo in the "Buttons" section now.
We don't need both.

* Pesto demo for the Flutter Gallery app.

* pesto.special.case

This demo burns battery continuously even when there's no change to the visual
appearance. This patch removes this demo to avoid giving the impression that
Flutter burns battery.

Related to #4120

*  Gallery demo start-time performance test

* move the material_gallery/demos.dart script

* rename all_demos.dart to all.dart

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

The 'routes' table is a point of confusion with new developers. By
providing a 'home' argument that sets the '/' route, we can delay the
point at which we teach developers about 'routes' until the point where
they want to have a second route.