* round 3

* partially address comments

* merge

* review notes

* review

* review

* review

1. Migrate simulator device log tailing to os_log toolchain
2. When the log tag (component) is available (iOS 11/Xcode 9), filter to
   the set of log lines with tag 'Flutter'.

As of iOS 11 / Xcode 9, Flutter engine logs are no longer recorded in the
simulator's syslog file, which we previously read using tail -f. Instead
they're now accessible through Apple's new macOS/iOS os_log facility,
via /usr/bin/log, which supports a relatively flexible query language.

When run in non-interactive mode, /usr/bin/log buffers its output in 4k
chunks, which is significantly smaller than what's emitted up to the
point where the observatory/diagnostics port information is logged. As a
workaround we force it to run in interactive mode via the script tool.

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

This reverts commit 8d07d3f6.

* Bump Dart SDK to 1.25.0-dev.9.0

* add link to sdk bug

* Guess sign with the first certificate when multiple are available in machine mode

* review

This reverts commit b2909a24.

This resubmits the following patches:

1. Use Xcode instruments to list devices (#10801)
Eliminates the dependency on idevice_id from libimobiledevice. Instead,
uses Xcode built-in functionality.

2. Make device discovery asynchronous (#10803)
Migrates DeviceDiscovery.devices and all device-specific lookup to be
asynchronous.

* Revert "Make device discovery asynchronous (#10803)"
This reverts commit 972be9c8.

* Revert "Use Xcode instruments to list devices (#10801)"
This reverts commit 37bb5f13.

This is to resolve a failure that looks related to a bad install of Xcode 8.0
on our build bots and should be reinstated when the infra issue is diagnosed
and resolved.

Instruments worked well when this was originally landed, and on the
following commit, but started failing two commits after this originally
landed. Manual invocation of instruments on the build host currently
results in:

```
dyld: Library not loaded: @rpath/InstrumentsAnalysisCore.framework/Versions/A/InstrumentsAnalysisCore
  Referenced from: /Applications/Xcode8.0.app/Contents/Developer/usr/bin/instruments
  Reason: image not found
Abort trap: 6
```

It appears the /Applications/Xcode8.0.app/Contents/Applications
directory (which contains Instruments) is missing on the host.

Migrates DeviceDiscovery.devices and all device-specific lookup to be
asynchronous.

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.

* remove usages of booted

* fix

Eliminates the need for the device/daemon code to get at the iOS/Android
tooling indirectly via Doctor. In tests, we now inject the workflow
objects (or mocks) directly.

For some reaosn, when we discovered our URI, we were re-instantiating
the `Completer` instance variable whose future we listen to in `nextUri()`.
This led to a race between a caller calling `nextUri()` and us discovering
the URI. If we happened to discover our URI before a caller called
`nextUri()`, then they would be left waiting on a future from the newly
allocated `Completer` (which would never complete).

Fixes #10064

This message will be picked up by IntelliJ and shown to the user in a toast.

`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

* Remove legacy .apk build.

Print out an error message telling the user to upgrade the project if
it's not Gradle-based. Removed all the obvious traces of the legacy
build.

Added support for Dart VM kernel snapshots in Gradle builds.

Fixed Android installs to verify that the app is actually installed, and
not just rely on the presence of the .sha1 file.

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

Only implemented for Android devices for now. Compare the installed SHA1
to the latest build. If they match, there's no reason to reinstall the
build.

Fixes #8295

* Move simulator screenshot logic to use simctl

* Add simulator screenshot tests

And fix some un-grammatical language below.

* Detects iPad 2 and iPad Retina as unsupported devices.
* Simplifies blacklisting logic.
* Minor improvements to error messages.
* Added unit tests.

This allows us, for example, to wait for the slow mode banner to have
been removed from the screen before triggering a screen shot.

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

...and add --diagnostic-port.

...and document port 0.