Support for thinning app frameworks to the target architecture was added
in 708909fc. This commit adds support
and error-checking for non-fat frameworks that are not of the target
architecture. In such cases, we now fail the build, and emit an error
message and the contents of lipo -info for the affected framework.

* Support thinning iOS frameworks to supported architectures

When building against frameworks that are distributed as
multi-architecture fat binaries, we want to strip the frameworks we
distribute down to only the architectures specified in $ARCHS.

This patch adds:
* The ability to specify commands to xcode_backend.sh (if none is
  specified, run BuildApp for backward compatibility).
* A 'thin' command that invokes lipo to thin down the distributed as
  described above.

* Add framework thinning step to iOS build

Invokes xcode_backend.sh thin on the build application.

* Limit architectures to arm64 in Xcode template

Flutter does not yet support armv7 iOS devices. Limit the $ARCHS build
variable to arm64 until then.

This ensures that accidental usages of dart:io's
file API don't creep in over time.

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.

Started failing at 0b737ac5.

* convert pubGet to throw ToolExit on non-zero exit code
* convert commandValidator to throw ToolExit for non-zero exit code
* convert flutter commands to throw ToolExit for non-zero exit code
* use convenience method throwToolExit
* only show "if this problem persists" for unusual exceptions

Fonts should never be bundled on Fuchsia.

This is causing issues when integratting Flutter into GN as the
generated depfile refers to snapshot as the target instead of the
bundle. We instead use a separate GN action to generate the
snapshot use the Flutter compiler to only assemble the bundle.

This patch makes it possible to build Flutter apps that use the framework on
Fuchsia.

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.

Previously, `flutter build` products were emitted to build/ and only
xcode build products were emitted to $SYMROOT.

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.

And fix a zillion omissions this uncovered.

This script has been subsumed by `flutter apk`.

Expose the main entry point for the tools via the library lets us run the tools
from the Flutter package, which simplifies the setup for end developers because
they don't need to declare a dependency on sky_tools directly.

I'm trying to get a feel for the code by writing some simple cleanup patches.

Add listen command and basic test, and don’t do unnecessary repeated work when listening or poking the android server.