Prep to get us ready to pull in a new dev SDK and bump our analyzer DEP.

* updates `crypto` (required by fresh analyzer)
* fixes newly flagged dead code warnings
* fixes switches that fall through and don't return

FAB segue animation is done in scaffold and the consts are there.

* send application files to the observatory's devfs

* update comment

Fix the switch to match the Material Design spec.

Our track and head were slightly off in size.
These now exactly match the sizes in
https://material.google.com/components/selection-controls.html#selection-controls-switch
We took the liberty of assuming the head overlaps the edge
of the track by 2 pixels (instead of the head and the track ending at the same point)
which the provided designs were conflicted about.

Huge thanks to @DaveShuckerow for noticing this issue!

@abarth

* Fix `flutter upgrade`

* Fix builds on iOS

Add missing returns.

As of `1.18.0-dev-0`, these cases will get flagged.  In the meantime, the

Enable launching applications on the iOS device with observatory and diagnostics server connected. (#4424)

This prevents multiple simultaneous runs of the analyzer from stomping
over each other (e.g. multiple runs of 'update-packages'). Certain
long-lived commands (like analyze, run, logs) are exempted once they've
done enough work to be safe from most stomping action.

This still doesn't make us entirely safe from craziness, e.g. if you're
half way through an 'update-packages' run and you call 'git pull', who
knows what state you'll end up in. But there's only so much one can do.

Fixes https://github.com/flutter/flutter/issues/2762

Add `@required` for some required parameters in `RRect`. Use elvis
operator to simplify a function.

* send supportsRestart flag when starting an app

* remove unused imports

* review comments

Also, make it clear the screen between results so it's more obvious
what's going on when you have new results (especially when you have
fixed everything).

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

Related to #4535

Fixes #4511

Instead of flinging to between 0.0 and 1.0, we should adapt the default
spring to the controller's upper and lower bounds.

Fixes #3545

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

This file is now app.dylib.

See https://github.com/flutter/flutter/issues/4438