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.
Adds an initial integration test that:
1. captures laid-out widget size
2. focuses a text field to show the keyboard and resize the view
3. unfocuses the text field to dismiss the keyboard and resize the view
4. verify that the final widget size matches the initial size
