Instead just disable the slider.

Fixes https://github.com/flutter/flutter/issues/10055

The slider never really made much sense and it violated UX guidelines.

https://github.com/flutter/flutter/issues/4147

Previously we used logical pixels. This made the accessibility metrics
tiny on modern devices, since the OS works in physical units.

Also add a bit more debugging info and some docs.

Previously we threw an exception to require a non-null value. Now we
just treat null as the common case of not stopping propagation.

Rather than complain about these in a code review I figured I'd just fix them. :-)

Fixes #10106

For some reaosn, when we discovered our URI, we were re-instantiating
the `Completer` instance variable whose future we listen to in `nextUri()`.
This led to a race between a caller calling `nextUri()` and us discovering
the URI. If we happened to discover our URI before a caller called
`nextUri()`, then they would be left waiting on a future from the newly
allocated `Completer` (which would never complete).

Fixes #10064

Requires https://github.com/flutter/engine/pull/3688

Also:

* Make TextPainter.preferredLineHeight honour root fontSize

* Remove bogus docs.

* More aggressively track dirty state for RenderEditable.

* Some tests.

* blind wrote everything except the user prompt

* works

* Add some logical refinements

* Make certificates unique and add more instructinos

* print more info

* Add test

* use string is empty

* review notes

* some formatting around commands

* add a newline

See https://github.com/flutter/flutter/pull/10000

* try use_to_and_as_if_applicable lint

* address review comments

If the caller doesn't explicitly give the brightness of the primary
color, we now estimate it using an algorithm from the Web Content
Accessibility Guidelines.

Also, this patch contains a function that converts RGB colors to
HSVColors. I was originally going to use that, but the WCAG algorithm
ended up seeming like a better choice. The patch still includes this
function because it's generally useful.

Fixes #5718

Fixes #9989

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

This way, you can test things even with a VerboseLogger.

During SDK version updates, output the commands used to tag the repo
with the SDK version.

The new assets have been optipng'ed.

Previously, we would strand the old route in the overlay even though it had
been removed from the history.

The default is topCenter, which is sensible, but someone might want to use a
bottomCenter alignment, for example.