
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.
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