Refactor DevFS so that it's easier to add new types of content such as kernel code
* add tests for DevFS package scanning
* add tests for DevFS over VMService protocol
* which covers _DevFSHttpWriter and ServiceProtocolDevFSOperations
* replace AssetBundleEntry and DevFSEntry with DevFSContent
* refactor to cleanup common code and replace some fields with locals
* rework .package file generation refactor away DevFSOperations.writeSource
* only update .package file if it has changed
* only write/delete/evict assets that have been changed/removed

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.

* 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.

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 makes the about page show the licenses of all the Dart packages that a Flutter app uses.

Issues that this does not yet resolve:
- I'm still working on getting the full list of licenses for the sky_engine package.
- Some of the licenses don't print very readably.
- There's no scrollbar on the license page.

I'll provide fixes for the first two in the coming days, but this should unblock anyone who is wanting to see something here, even if it's not quite complete. :-)

----

The patch makes the following changes: 

- The license registry is now asynchronous, since the data comes from disk.
- I moved the default license collector from the foundation package to the services package since it uses the default asset bundle now.
- The FLX builder now includes the LICENSE files of each Dart package mentioned in the `.packages` file.

This is now obsolete

* Add a map of relative to absolute paths for assets located outside the app's
  source directory
* If a "packages" directory exists, obtain assets from there instead of
  using the packages/ prefix to indicate package map lookup

* refactor the --resident run option into a separate file

* update daemon to run --resident apps

* re-plumbing daemon start

* send app logs

* update tests

* review changes

* fix test runner

* remove PackageMap.createGlobalInstance; rely on the ctor

* review comments

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.

* complain on missing entries in flutter.yaml

* change to a hard failure

This currently requires a local build of the engine.

Run "flutter build aot" to build an AOT snapshot.
Run "flutter build apk --profile" to build a "profile mode" APK that uses AOT.

* exclude files from build/ when looking for asset variants

* exclude some dirs when creating flx files

Any font asset referenced in the fonts section of flutter.yaml will now be
included in the FLX assets.  It's no longer necessary to separately list the
font in the assets section.

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