* send channel name as a custom dimension

* tweaks from review

* remove unused import

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

This libimobiledevice tool is no longer used anywhere in the flutter_tools codebase.

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.

Fixes https://github.com/flutter/flutter/issues/10681

Fixes https://github.com/flutter/flutter/issues/10630

* move getTransformTo method from RenderBox to RenderObject.

* Cleanup comment. Add slivers test.

* 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.

Eliminates the dependency on idevice_id from libimobiledevice. Instead,
uses Xcode built-in functionality.

This should improve debug-time performance.

Moves all remaining calls to tools that are part of the libimobiledevice
suite of tools to the IMobileDevice class. This allows for better
tracking of this dependency, and easier mocking in tests.

None of these is used outside of devices.dart and being public covers up
analysis warnings when they're unused.

All invocations should go via the IMobileDevice class in mac.dart.

See https://github.com/flutter/flutter/issues/10701

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.

Extract out IMobileDevice class, move class to idevice_id, ideviceinfo
(and eventually other libimobiledevice tools such as iproxy) behind this
interface.

Add tests for the case where libimobiledevice is not installed, the case
where it returns no devices, and the case where it returns device IDs.

* 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.

- remove unused collector parameter
- clarify a comment
- inline _currentPackageTestDir

This code is unused in any test. In upcoming changes that migrate to
Xcode instruments based device listing, we'll mock out the instruments
output separately.