* Revert "Revert "Reland pass filesystem options (#15622)" (#15651)"

This reverts commit 0f3eada0.

* Fix for windows failing test

* !isEmpty -> isNotEmpty

This reverts commit bbcf55a5 as it breaks hot reload on Windows.

* Revert "Revert "Pass filesystem-root, filesystem-scheme options through `flutter run, build` and gradle. (#15592)" (#15621)"

This reverts commit c663e336.

* Check whether filesystem-root is emtpy.

* Restore broken merge

* Fix windows test

* Adjust regex in test. Invalidate files by file path.

Revert "Pass filesystem-root, filesystem-scheme options through `flutter run, build` and gradle. (#15592)" (#15621)

This reverts commit 0f659fb1 as it
breaks bin/tasks/gradle_plugin_test.dart.

* Pass filesystem-root, filesystem-scheme options through flutter run/build/gradle.

* Put generated kernel files into <build> directory

* USe default value for kernel file

* Plumb a --strong option through to the front end server and the engine
so that we can run flutter apps in preview-dart-2 and strong mode

* - Address analyzer lint issues
*- correctly set up strong mode option in the case of AOT builds

This adds --preview-dart-2 flag that enables use of Dart 2.0 Frontend in Flutter tools.

* Remove legacy .apk build.

Print out an error message telling the user to upgrade the project if
it's not Gradle-based. Removed all the obvious traces of the legacy
build.

Added support for Dart VM kernel snapshots in Gradle builds.

Fixed Android installs to verify that the app is actually installed, and
not just rely on the presence of the .sha1 file.

* Remove the workaround that pinned args to v0.13.6
This reverts most of the changes in commit 6331b6c8
* throw exception if exit code is not an integer
* rework command infrastructure to throw ToolExit when non-zero exitCode
* convert commands to return Future<Null>
* cleanup remaining commands to use throwToolExit for non-zero exit code
* remove isUnusual exception message
* add type annotations for updated args package

* 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

* show hidden flags when showing verbose help
* flutter -v show verbose help

* refactor _run to runCmd

* replace requiresProjectRoot getter with call to commandValidator

* replace requiresDevice getter with call to findTargetDevice

* trace command requires a debug connection, not a device

* inline androidOnly getter

* rename command methods to verifyTheRunCmd and runCmd

* move common verification into BuildSubCommand

* rename deviceForCommand to device

* rename methods to verifyThenRunCommand and runCommand

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.

Also, add a "flutter build flx --report-licensed-packages" option for
when you need to get the list of the packages affected by licenses.

* This reverts commit 5e7e7b6e.

* default to lib/main.dart

This reverts commit 32ad81c8.

Host tools can be found in the artifact cache directory for the host platform.
If a developer wants to use a local engine build instead, then provide an
--engine-build flag that selects the specific engine build variant.

This allows building an FLX without leaving any temporary files in the app's
source directory

Fixes #1942.

Rather than managing all the Material Design icons manually, we now
manage them using an icon font. The icon font contains glyphs for each
icon in an efficient vector format.

This patch updates the FLX tooling to include the MaterialIcons font and
updates the Icon widget to use the font instead of asset images.

Fixes #2313
Fixes #2218
Fixes #2009
Fixes #994

This splits flx.build() into two methods, flx.build() and
flx.assemble().  builD() now does the following:
1) constructs the manifest map by reading the manifest from the
   file system
2) "compiles" the dart code into the snapshot file
3) Invokes assemble()

This allows external build toolchains to construct their own
manifest map (possibly using a different manifest syntax)
and create their own snapshot file