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

This patch sets a default path for Flutter.framework that's sensible based on
the FLUTTER_MODE.

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.

This patch adds some sensible defaults to xcode_backend.sh and documents how to
build hello_services for iOS using Xcode.

Fixes #4298

Instead of embedding Flutter.framework and BuildFlutterApp in the Xcode
project, we store these in flutter.git, where we can reference them.