* prefer const constructors

* fix double throw

* eliminate sky snapshot

* roll engine

This should make our scripts compatible with PowerShell 2 or newer. PowerShell 2 was released in October 2009 and shipped with Windows 7 as default. (I suspect the scripts are now also compatible with PowerShell 1, but that's unconfirmed). This fixes #8606.

The PR also introduces better error handling when Flutter fail to download the Dart SDK to fix #8627.

This is consistent with the rest of the iOS toolchain.

* Revert "Update Hello World to latest iOS project template (#8634)"

This reverts commit 6dc6ed4e.

* Revert "Update Gallery to latest iOS project template (#8637)"

This reverts commit 84c42aac.

* Revert "Update Stocks to latest iOS project template (#8640)"

This reverts commit 90774c18.

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.

Already enabled on Android.

Update templates and example Xcode project files to indicate they've
been tested for Xcode 8.3 compatibility.

This implements the `DartDependencySetBuilder` completely in Dart instead of calling out to `sky_snapshot` (Linux/Mac) or `gen_snapshot` (Windows) and allows us to use the same code path on all supported host platforms.

It also slightly reduces hot reload times on Linux from ~750ms to ~690ms for the unchanged flutter_gallery app and significantly reduces hot reload times on Windows from almost 1.5s to just slightly slower than on Linux.

This change will also allow us to retire `sky_snapshot` completely in the future.

Gallery and stocks:
* Add 20x20 notification icons.
* Remove unused 76x76 (3x scale) icon.

Other examples: update JSON to Xcode 8.2.1 defaults

Fixes warnings for missing/unused icon assets in Xcode 8.2.x

Fixes #8562

* for Win 7, we need to import BitsTransfer before using it
* switch to PowerShell's build-in archive expander (instead of relying on .NET)

This adds the test harness for record/replay tests.

* Request the user upgrade if the Android SDK version is 24

* Update android_sdk_test.dart

* Update android_sdk.dart

This matches the default for Runner.xcodeproj and fixes a linker warning
when the two projects use different values.

Add 20x20 notification icons.
Remove unused 76x76 (3x scale) icon.
Fixes warnings for missing/unused icon assets in Xcode 8.2.x

This prevents project re-validation on initial Xcode launch.

1. Add matchers for the `ProcessExit` exception class
2. Add ability to control the setup of the `AppContext` we use in
   `testUsingContext()`
3. Clean up the code that figures out the location of `Cache.flutterRoot`
   such that it works with `pub run test`. It previously only worked
   when the tests were invoked with standalone `dart`

`#3` above will also help unblock #7941

* Small update to instructional comments in app template

* Update Flutter Driver comment

Good suggestion to add the link, Seth! I also updated the working to match what that page uses.

Engine roll to include https://github.com/flutter/engine/pull/3462