* enable lint prefer_asserts_in_initializer_lists

* enable --assert-initializer

This makes command validation happen as part of `verifyThenRunCommand()`,
using a newly introduced protected method (`validateCommand()`) rather than
a `commandValidator` property (that subclasses were responsible for manually
invoking).

This reverts commit 8d07d3f6.

* Guess sign with the first certificate when multiple are available in machine mode

* review

The main purpose of this PR is to make it so that when you set the
initial route and it's a hierarchical route (e.g. `/a/b/c`), it
implies multiple pushes, one for each step of the route (so in that
case, `/`, `/a`, `/a/b`, and `/a/b/c`, in that order). If any of those
routes don't exist, it falls back to '/'.

As part of doing that, I:

 * Changed the default for MaterialApp.initialRoute to honor the
   actual initial route.

 * Added a MaterialApp.onUnknownRoute for handling bad routes.

 * Added a feature to flutter_driver that allows the host test script
   and the device test app to communicate.

 * Added a test to make sure `flutter drive --route` works.
   (Hopefully that will also prove `flutter run --route` works, though
   this isn't testing the `flutter` tool's side of that. My main
   concern is over whether the engine side works.)

 * Fixed `flutter drive` to output the right target file name.

 * Changed how the stocks app represents its data, so that we can
   show a page for a stock before we know if it exists.

 * Made it possible to show a stock page that doesn't exist. It shows
   a progress indicator if we're loading the data, or else shows a
   message saying it doesn't exist.

 * Changed the pathing structure of routes in stocks to work more
   sanely.

 * Made search in the stocks app actually work (before it only worked
   if we happened to accidentally trigger a rebuild). Added a test.

 * Replaced some custom code in the stocks app with a BackButton.

 * Added a "color" feature to BackButton to support the stocks use case.

 * Spaced out the ErrorWidget text a bit more.

 * Added `RouteSettings.copyWith`, which I ended up not using.

 * Improved the error messages around routing.

While I was in some files I made a few formatting fixes, fixed some
code health issues, and also removed `flaky: true` from some devicelab
tests that have been stable for a while. Also added some documentation
here and there.

`adb` can sometimes hang, which will in turn hang the Dart isolate if
we're using `Process.runSync()`. This changes many of the `Device` methods
to return `Future<T>` in order to allow them to use the async process
methods. A future change will add timeouts to the associated calls so
that we can properly alert the user to the hung `adb` process.

This is work towards #7102, #9567

* Rename `Device.platform` to `Device.targetPlatform` to avoid
  collision with the exported variable from `platform.dart`

... and run some more tests on Windows.

This remove a very brittle aspect of flutter drive, whereby it would
assume a known port instead of explicitly finding out what it was.

Fixes #7692 and hopefully fixes the devicelab tests.

Move the drive/run common stuff to RunCommandBase, rather than the
superclass which applies to everything.

Implement --no-resident (fixes
https://github.com/flutter/flutter/issues/5448). This is particularly
useful when running flutter from scripts.

Rename RunAndStayResident to ColdRunner.

...for the port arguments

...and add --diagnostic-port.

...and document port 0.

See https://github.com/dart-lang/sdk/issues/28463, which I think is a
regression.

This also fixes the regression introduced by
https://codereview.chromium.org/2559773002 whereby we were no longer
checking any of the lints.

This has been fixed: https://github.com/dart-lang/linter/issues/317

* Allow for application-specific log readers.

When running with prebuilt application binaries, those applications
aren't guaranteed to be named "Runner" (as it is when we build
the app locally in Flutter tools)

* Remove the workaround that pinned args to v0.13.6
This reverts most of the changes in commit 6331b6c8
* throw exception if exit code is not an integer
* rework command infrastructure to throw ToolExit when non-zero exitCode
* convert commands to return Future<Null>
* cleanup remaining commands to use throwToolExit for non-zero exit code
* remove isUnusual exception message
* add type annotations for updated args package

* convert pubGet to throw ToolExit on non-zero exit code
* convert commandValidator to throw ToolExit for non-zero exit code
* convert flutter commands to throw ToolExit for non-zero exit code
* use convenience method throwToolExit
* only show "if this problem persists" for unusual exceptions

Fixes #4261

* refactor _run to runCmd

* replace requiresProjectRoot getter with call to commandValidator

* replace requiresDevice getter with call to findTargetDevice

* trace command requires a debug connection, not a device

* inline androidOnly getter

* rename command methods to verifyTheRunCmd and runCmd

* move common verification into BuildSubCommand

* rename deviceForCommand to device

* rename methods to verifyThenRunCommand and runCommand

... in place of manual process wrangling.

Fixes flutter drive breakage introduced in d2fda677.

* improve device matching error message
fixes https://github.com/flutter/flutter/issues/5675

* This reverts commit 5e7e7b6e.

* default to lib/main.dart