This adds --preview-dart-2 flag that enables use of Dart 2.0 Frontend in Flutter tools.

* 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 8d07d3f6.

This reverts commit 5f9acc41.

...so that we can compare it to the not-working test next to it.

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.

Replace use of ideviceinfo in devicelab tests with Xcode instruments
lookup.

* Benchmark for semantic overhead during transitions

* review comments

* resurrect analyzer benchmarks

* move analyzer_benchmark to the more stable linux/android

* report average rather than best result

Update Android package name.

Since lib/main.dart does not exist, update test to explicitly build lib/keyboard_resize.dart.

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

Enable plugin infra for both iOS and Android post-#9791.

Brings hello_world and flutter_gallery Android code in line with the
corresponding iOS code.

Eliminates Gallery-specific hardcoding of app package and activity name
in the test.

Fixes breakage introduced with 1dc2d4eb
due to main activity class now being MainActivity.

Remove checked in .lock, remove checked in cocoapods scripts in xcode project, let the automated test run pod install specifically for this test

Broken by 89af5ec8.

* comprehensive list of lints
* add comments to commented out lint
* fix unnecessary_this lints
* exclude prefer_final_fields

Aggressively apply the const lint.