* Change some of the dev/devicelab tests to support testing against local engine.

We can already configure flutter tools to use local engine by setting
FLUTTER_ENGINE environment variable. However when this variable is set
this also requires setting --local-engine to specify which flavor of
engine to use.

This change changes tests in dev/devicelab to pass a sensible default for
--local-engine, e.g. when testing hot reload on Android we pass android_debug
and when testing release AOT build for IOS we pass ios_release.

* Fix analysis issues

* Update utils.dart

* Restructure hot mode test so it runs interactively.

This allows to add a benchmark for hot reload after actual source code change.

* Add curly braces, refactory copyRecursive

* Revert "Revert "fire service protocol events for frames (#11565)" (#11727)"

This reverts commit f25e2f52.

* move the postEvent() call into a separate method

This reverts commit 5f9acc41.

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.

* resurrect analyzer benchmarks

* move analyzer_benchmark to the more stable linux/android

* report average rather than best result

Aggressively apply the const lint.

* devicelab benchmarks run on Windows

* fix analyzer issue

* fix test

* fix pubspec

...now that we have generic methods, their types need to be specified too.

* fix flutter watch benchmark

Syncs https://github.com/flutter/cocoon/commit/d2d7950ecd2c72421d75d9b5d54805cc554e92a8

* Split Android/iOS impl behind a unified interface

Syncs https://github.com/flutter/cocoon/commit/db87e10fa54317115479d0b85280c7e5eed08ff7

* Switch from pub get to flutter packages get

Syncs https://github.com/flutter/cocoon/commit/b378005cbbc5330058d5240a2970ca477c2c9722

* "silent" option in test runner; fix analysis errors;