This reverts commit 8d07d3f6.

* blind wrote everything except the user prompt

* works

* Add some logical refinements

* Make certificates unique and add more instructinos

* print more info

* Add test

* use string is empty

* review notes

* some formatting around commands

* add a newline

* Remove all initial cocoapod traces from flutter create except Podfile

* Make updateXcodeGeneratedProperties parameters named

* review notes

* 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

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.

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.

Artifacts are now located in a central place.
This will enable us to downlaod artifacts when we need them (instead of
downloading them all upfront).
This also makes replacing sky_snapshot with gen_snapshot easier.

On flutter run, we update ios/Flutter/Generated.xcconfig with various
Flutter-specific settings required by xcode_backend.sh during a build
from Xcode. These settings need to be present at the time the project is
loaded since Xcode doesn't pick up live updates to these files.

Without these settings, Xcode fails to locate xcode_backend.sh itself,
causing the build to fail until the Xcode project has been closed and
re-opened. This also prevents Xcode's project updater from 'helpfully'
suggesting to clean up and delete the Generated.xcconfig file.

This removes direct file access from within flutter_tools
in favor of using `package:file` via a `FileSystem` that's
accessed via the `ApplicationContext`.

This lays the groundwork for us to be able to easily swap
out the underlying file system when running Flutter tools,
which will be used to provide a record/replay file system,
analogous to what we have for process invocations.

As of Xcode 7, Apple recommends setting CFBundleIdentifier to
$(PRODUCT_BUNDLE_IDENTIFIER).

This change adds a top-level getBuildDirectory func and funcs for
android, aot, asset, ios build products.

Developers may now add a "build-dir" mapping to their
~/.flutter_settings (JSON format) config file. Output directory is
relative to the main flutter application directory.

This change also changes the default build directory for iOS builds to a
subdirectory of the configured build directory, 'build/ios' by default.

This patch changes the default `flutter create` command to generate a
HelloServices-style app on iOS. As a consequence, the Atom integration now
fully works with HelloServices.

The `flutter.buildMode` property now controls whether to build in debug,
profiling, or release.

Add a FLUTTER_MODE variable to the generated Xcconfig and fix snapshot generation on all targets. (#4362)

Get rid of the DART_EXPERIMENTAL_INTERPRETER_FLAG.

Fixes #4298

This patch updates how we generate xcconfig files to match what
xcode_backend.sh expects.

Fixes #4186

Wire up ios-profile and ios-release. Switching between debug, profile and release starts the build process from scratch. (#4063)

Artifacts from local engine builds will be found based on the --local-engine flag

* fix analysis errors

* review comments; fix test

* re-add an export for debugPrint

* rename the --develop option to --debug

* fail if both --debug and --deploy are specified

* use the new cache code to locate more artifacts

* add todo

* review comments

In case the user changes the path of the Flutter project on disk, the Dart SDK or the Flutter repository itself, the old Xcode paths would be invalid. Now, we rewrite them when we build the project.