Fixes #8618

Also, some renames for more accuracy.

* do not forward status if stream is closed
* remove devfs test timeout

These symbols were not intended to be public. Also, remove some bogus dartdocs
for PhysicalModeLayer.

(follow-on to #8698)

Changing to an inline function, making the type more clear.

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

`all.dart` is no longer needed. Furthermore, it causes tests to
be skipped, or to silently fail to run anything.

Fixes #7941

Use line-based iteration in place of word-based iteration over the
framework directories, to avoid breaking on spaces.

* Revert "Eliminate CocoaPods install step (#8694)"

This reverts commit f4a13bc7.

If the developer is relying on CocoaPods and hasn't done a pod install, we will do it for them. This is needed for a smooth native plugin experience, similar to what Gradle is doing on the Android side.

There's no hard dependency on CocoaPods. We only run pod install if the project uses CocoaPods, so developers are still free to use alternatives if they prefer (and if they don't want to use native plugins).

Fixes #8685
Fixes #8657
Fixes #8526

* Require CocoaPods 1.0.0 or newer.

And make sure we don't get a crash if running `pod install` fails.

* Address review feedback

This avoids unintentionally installing an older version if the
developer's homebrew installation is out-of-date.

The flutter tap was deleted as part of the resolution to #7783.

If the developer is relying on CocoaPods and hasn't done a pod install,
they'll get a build failure indicating the issue.

This also avoids a hard dependency on CocoaPods in the tool and allows
developers to customize their Xcode steps to use alternatives such as
Carthage if they prefer.

As of the latest Xcode versions, the latest published libimobiledevice
is out-of-date and build from HEAD is required.

This fixes two bugs:
1. Update initial install instructions to add --HEAD flag.
2. Update uninstall, reinstall instructions to include
   --ignore-dependencies flag, since other brew formulae depend on
   libimobiledevice.

Homebrew is an expected part of the iOS toolchain. Update the error
message to assume that.

* prefer const constructors

* fix double throw

* eliminate sky snapshot

* roll engine

This is consistent with the rest of the iOS toolchain.

Since iOS builds are CocoaPods enabled by default, we should make sure to run `pod install` to get pods wired up before building the app.

Also added a check to `flutter doctor` to verify CocoaPods is installed.

I'm passing FLUTTER_FRAMEWORK_DIR to the `pod install` command, so we can have the app's Podfile link in Flutter.framework as a pod instead of having to copy it over in xcode_backend.sh.