* fix some bad indentations

* remove suspicious line

This converts existing ## Sample code samples to {@tool sample}...{@end-tool} form.

Also:
1. Fixed a minor bug in analyze-sample-code.dart
2. Made the snippet tool only insert descriptions if the description is non-empty.
3. Moved the Card diagram to before the code sample.

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

* lint unnecessary_new on samples

* fix tests

fuchsia_tester.dart still assumes Dart 1. Previously, it ran tests directly
from source, flutter_platform.dart automatically runs a kernel compile when
operating in Dart 2 mode, but this assumes a functional Dart SDK is available
in the artifacts directly, and fuchsia_tester.dart mocks out the artifacts
directory with an empty temp dir.

Remaining work is:
1. Get the frontend server building as a dependency on Fuchsia.
2. Patch fuchsia_tester.dart to use a valid Dart SDK and frontend server.

This also reverts migration to Dart 2 typedef syntax.

This reverts commit 6c56bb24. (#18362)
This reverts commit 3daebd05. (#18316)

Now that Dart 1 is turned off, reapplying my change to turn on the prefer_generic_function_type_aliases analysis option, and fix all the typedefs to Dart 2 preferred syntax.

Also eliminated the unused analysis_options_repo.yaml file and turned on public_member_api_docs in analysys_options.yaml.

No logic changes, just changing the typedef syntax for all typedefs, and updating analysis options.

Revert "Update typedef syntax to use Function notation and turn on lint for old notation. (#18035)" (#18041)

This reverts commit 32586020.

Fixes #18028

Just changes typedef declarations, no logic changes.

* Move the splitting of licenses to an isolate

This improves (from horrific to terrible) the performance of the
license screen. It also introduces a feature in the foundation layer
to make using isolates for one-off computations easier.

The real problem that remains with this, though, is that transfering
data between isolates is a stop-the-world operation and can take an
absurd amount of time (far more than a few milliseconds), so we still
skip frames.

More work thus remains to be done.

* - Add profile instrumentation to the isolate compute() method
- Add profile instrumentation to the LicensePage
- Add profile instrumentation to the scheduleTask method
- Make scheduleTask support returning a value
- Make the license page builder logic use scheduled tasks so that it doesn't blow the frame budget

Covers lib/ in package:flutter.

Also, fix a few minor bugs found by the test.

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.