This improves the documentation of the `compute()` function as follows:

  * Instead of making `compute` a top-level constant, we make it a
    function. This allows the generated API docs to show a function
    signature with parameters and their names, making it *much* clearer
    to users what function is being documented.

  * We mention that on web-backends this is running `compute()` on the
    normal eventloop whereas on on native platforms it runs in a
    separate isolate.

  * We mention that callback, message and result have to be sendable
    across isolates. We also mention that they may be copied.

  * We link to both `Isolate.run` & `SendPort.send` for more information.

  * We use `M` for the type of `message` instead the rather confusing
    `Q`.

* migrate foundation to nullsafety

* address review comments

* add language version 2.8 in packages/flutter

* enable non-nullable analyzer flag

* Update project.pbxproj files to say Flutter rather than Chromium

Also, the templates now have an empty organization so that we don't cause people to give their apps a Flutter copyright.

* Update the copyright notice checker to require a standard notice on all files

* Update copyrights on Dart files. (This was a mechanical commit.)

* Fix weird license headers on Dart files that deviate from our conventions; relicense Shrine.

Some were already marked "The Flutter Authors", not clear why. Their
dates have been normalized. Some were missing the blank line after the
license. Some were randomly different in trivial ways for no apparent
reason (e.g. missing the trailing period).

* Clean up the copyrights in non-Dart files. (Manual edits.)

Also, make sure templates don't have copyrights.

* Fix some more ORGANIZATIONNAMEs

This reverts commit 90500a5d.