This allows `flutter run` to work on hosts that are IPv6-only.

Add hidden options --extra-front-end-options and --extra-gen-snapshot-options to flutter tool (#12219)

This CL introduces 2 hidden options to 'flutter build aot' and 'flutter run' for passing arbitrary arguments to front-end server and to gen_snapshot tool when building and running flutter app in --profile or --release modes.

The ability to pass arbitrary options simplifies various experiments, as it removes the need to change defaults and rebuild flutter engine for every tested configuration.

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).

* round 3

* partially address comments

* merge

* review notes

* review

* review

* review

* Add --trace-skia parameter to flutter run

Skia tracing is extremely useful for internal debug, but reduces the
amount of space available in the Dart Timeline buffers.
Disable skia tracing by default and expose them via the --trace-skia
flag.

* Roll Engine to 57a1445a45964d386500c39f5e8d06db060abadb

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

This reverts commit 8d07d3f6.

* report run target and if it is an emulator

* don't print debug

* rename parameter, remove unused variable

* fix test

* fix comment

* tweak from review, and fix analyzer error

* send custom parameters for the event, not the session

* fix mock

* use the +1 for usage

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.

Use a top-level getter in mac.dart rather than a static instance getter
and a top-level getter in ios_workflow.dart. Makes this code consistent
with how we do context lookups elsewhere.

FlutterCommand.runCommand subclasses can optionally return a FlutterCommandResult which is used to append additional analytics. 

Fix flutter run timing report and add a bunch of dimensional data

`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

* Fix tests to use Ahem, and helpful changes around that

- Fix fonts that had metric-specific behaviours.

- LiveTestWidgetsFlutterBinding.allowAllFrames has been renamed
  to LiveTestWidgetsFlutterBinding.framePolicy.

- LiveTestWidgetsFlutterBinding now defaults to using a frame policy
  that pumps slightly more frames, to animate the pointer crosshairs.

- Added "flutter run --use-test-fonts" to enable Ahem on devices.

- Changed how idle() works to be more effective in live mode.

- Display the test name in live mode (unless ahem fonts are enabled).

- Added a toString to TextSelectionPoint.

- Style nit fixes.

* Roll engine to get Ahem changes.

* Update tests for dartdoc changes.

* Fix flutter_tools tests

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

Fixes #8209

Ref: https://developer.apple.com/xcode/

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.

* pass additional named params to daemon run

* add a trailing comma

This ensures that accidental usages of dart:io's
file API don't creep in over time.

This removes direct file access from within flutter_tools
in favor of using `package:file` via a `FileSystem` that's
accessed via the `ApplicationContext`.

This lays the groundwork for us to be able to easily swap
out the underlying file system when running Flutter tools,
which will be used to provide a record/replay file system,
analogous to what we have for process invocations.

* have flutter run --machine log to stdout

* add a test

* add docs; fix type annotation lint

* 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

- [x] Remove old benchmark code from the non-hot runner.
- [x] Remove some dead code.
- [x] Fix a throw that should be a rethrow.