(second attempt)

This reverts commit 4f1b6604.

* [tools] consolidate "preview Dart 2" option in BuildInfo

* format BuildInfo constructor

* Plumb a --strong option through to the front end server and the engine
so that we can run flutter apps in preview-dart-2 and strong mode

* - Address analyzer lint issues
*- correctly set up strong mode option in the case of AOT builds

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

* executable.dart#main() depends on runner.dart#run()
* Refactor code such that non-commands don't depend on commands.

No code was actually changed in this PR - code was merely moved from
point A to point B.

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

* fix some call sites of printError

* review comments

`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

* improve progress display when running apps; speed up startup

* review comments

* 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

Fixes https://github.com/flutter/flutter/issues/6849

Function keys don't work great on any platform we support:
* Mac doesn't have first-class function keys.
* On Ubuntu: F1 opens the system help and F10 opens the file dialog.
* ... and Windows is a mess as well.

- [x] Don't use package:stack_trace.
- [x] Don't use Chain.capture.
- [x] Fix an instance of aggressive catching of exceptions

Related #8742

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

Instead, actually exit the tool.

Fixes #8363.

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.

- [x] Wire up dependency checker and plumb flags down to the right place

Fixes #7014

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.

* reapply handle new Observatory URL changes
Fixes https://github.com/flutter/flutter/issues/6843

* allow non numeric IPv4 addresses
Fixes https://github.com/flutter/flutter/issues/7051

https://github.com/flutter/flutter/commit/737a55ef3e47036220e72f3b3e17353d461b5fab

- [x] Make iOS and Android application startup paths consisitent.
- [x] Remove the now unncessary Android specific special cases from the runners.

* Allow for application-specific log readers.

When running with prebuilt application binaries, those applications
aren't guaranteed to be named "Runner" (as it is when we build
the app locally in Flutter tools)

* 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.

* make showElapsedTime default to true

* support nested progresses

* improve hot reload message

* rethrow