A recreate of #11669 by
recopying all flutter/flutter stuff from scratch, and cherry picking all
of my subsequent commits.

This is a single super mega PR attempting to migrate all Material and
Cupertino code from flutter/flutter to material_ui and cupertino_ui.

### How the PR was created

```
git filter-repo --path packages/flutter/lib/src/material --path packages/flutter/test/material --path examples/api/lib/material --path examples/api/test/material --path packages/flutter/lib/src/cupertino --path packages/flutter/test/cupertino --path examples/api/lib/cupertino --path examples/api/test/cupertino --path packages/flutter/lib/fix_data/fix_material --path packages/flutter/test_fixes/material --path packages/flutter/lib/fix_data/fix_cupertino.yaml --path packages/flutter/test_fixes/cupertino 
cd ../packages
git remote add source-origin ../flutter
git fetch source-origin
git merge source-origin/master --allow-unrelated-histories
```

I then cherry picked everything I could from the previous PR
(#11669), skipping merge
commits.

### What was included in the copy

 * The main source code (packages/flutter/lib/src/<material/cupertino>).
 * The widget/unit tests (packages/flutter/test/<material/cupertino>).
 * The examples (examples/api/lib/<material/cupertino>).
 * The example tests (examples/api/test/<material/cupertino>).
 * The dart fixes (packages/flutter/lib/fix_data).
 * The dart fix tests (packages/flutter/test_fixes).

### TODOs

 * [x] Do exports for Material and Cupertino.
* [x] Confirm the directory structure (current plan: lib/src/ for now,
reorganize after 1.0.0).
 * [x] Try installing and using the packages locally.
* [x] Try running the tests locally (works except for tests with cross
imports).
* [x] Try running the example tests locally (work after migrating
`package:flutter_api_samples` import).
 * [x] Verify examples work.
* [x] Verify that `git blame` appears the same in both repos on various
files. (Yes, but the SHAs are different, as expected.)
 * [x] Add auxiliary files for dart fixes and tests.
* [x] Port imports of material/cupertino in places like examples/test to
use the new packages.
 * [x] ~~Fix~~ Skip all test cross imports.
* [x] Make sure the analyzer is working in the same exact way as it does
in flutter/flutter. (#11692)

### Open questions

 * ~~Are we sure about the directory structure?~~
   * Confirmed with @dkwingsmt and @QuncCccccc.
* Anything else that should be ported along with this? If these commits
touch files that are not included in this PR, but that we later port in,
the commits will likely have different SHAs.
* I'm hoping we got it all at this point but if anyone notices anything
let me know.
 * ~~Do we like the examples setup?~~
* Worked with @stuartmorgan-g and @chunhtai to get this to match the
other packages in this repo.
* Are we cool with skipping cross imports tests and fixing them later?
This approach should not fracture commits because the commits were made
in flutter/flutter after the code was copied.
   * Tests are in the temporarily_skipped_tests folder.

### How to try using these packages

Add the local packages to your project:

```
flutter pub add cupertino_ui --path ../../packages/packages/cupertino_ui
flutter pub add material_ui --path ../../packages/packages/material_ui  
```

Then import the packages instead of the libraries in the SDK:

```diff
- import 'package:flutter/material.dart';
- import 'package:flutter/cupertino.dart';
+ import 'package:material_ui/material_ui.dart';
+ import 'package:cupertino_ui/cupertino_ui.dart';
```

### How to review this PR

 * Do read the PR description at the top, including Open Questions.
* Don't read all 600k+ lines changed! Ignore the main source code and
test files. The only thing I changed in there is to change imports from
`flutter/material.dart` to `material_ui/material_ui.dart` and the
equivalent for Cupertino.
* Do look closely at the file tree browser on the left side of the
"Files Changed" tab, though. Check the directory structure of
material_ui and cupertino_ui and make sure it looks good to you.
* Do take a look at the small config files like pubspecs,
analysis_options, etc.
* See the examples and their tests now located in
material_ui/material_ui_examples and cupertino_ui/cupertino_ui_examples.
* See the data driven fixes in lib/fix_data and their tests in
test_fixes.

And let me know if there is any code that's not in this PR that you
expected to be. It will be best to get everything we can in this one PR
rather than following up later in order to prevent fragmenting commits.

### Resources

 * Closed PR where i tried to import everything but forgot dart fixes.
    - #11568
* Closed PR where I tried and failed to import a single widget by
individually importing all of its transitive dependencies with `git
filter-repo` commands.
   * #11526
* Closed PR where I tried to import just colors.dart (no dependencies),
but found that it would create different SHAs between it and this mega
PR.
   * #11543
* Closed PR where I tried to import all of Material, before realizing
that I would create different SHAs between that PR and a future
Cupertino PR.
   * #11549
* Closed main big PR with most of the history:
#11669
* Closed PR where I tried to cherry pick everything:
#11887