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

* clamp the pixels used in the page calculation to be within min + max extents to avoid pages outside of the range of children

* add test to check onPageChanged not fired on overscroll

Fix a few regressions.

Covers lib/ in package:flutter.

* add crash reporting without enabling it

* do not drop futures on the floor

* return exitCode from executable run

* debug travis

* remove unnecessary todo

* rename local fs getter