Publish web_embedding (#1777)

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-devrel
channel on [Discord].

<!-- Links -->
[Flutter Style Guide]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[CLA]: https://cla.developers.google.com/
[Discord]: https://github.com/flutter/flutter/wiki/Chat
[Contributors Guide]:
https://github.com/flutter/samples/blob/main/CONTRIBUTING.md


Co-authored-by: Mark Thompson
<[email protected]>
Co-authored-by: David Iglesias <[email protected]>

Co-authored-by: Mark Thompson <[email protected]>
Co-authored-by: David Iglesias <[email protected]>

Author: 3 people

web_embedding/README.md

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+# web_embedding
+
+This directory contains examples of how to embed Flutter in web apps (without iframes):
+
+* **element_embedding_demo**: Modifies the index.html of a flutter app so it is
+  launched in a custom `hostElement`. This is the most basic embedding example.
+* **ng-flutter**: A simple Angular app (and component) that replicates the above
+  example, but in an Angular style.
+
+Check the `README.md` of each example for more details on how to run it, and the
+"Points of Interest" it may contain.

web_embedding/element_embedding_demo/.gitignore

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+# Miscellaneous
+*.class
+*.log
+*.pyc
+*.swp
+.DS_Store
+.atom/
+.buildlog/
+.history
+.svn/
+migrate_working_dir/
+
+# Keeping the repo
+.metadata
+pubspec.lock
+
+# IntelliJ related
+*.iml
+*.ipr
+*.iws
+.idea/
+
+# The .vscode folder contains launch configuration and tasks you configure in
+# VS Code which you may wish to be included in version control, so this line
+# is commented out by default.
+#.vscode/
+
+# Flutter/Dart/Pub related
+**/doc/api/
+**/ios/Flutter/.last_build_id
+.dart_tool/
+.flutter-plugins
+.flutter-plugins-dependencies
+.packages
+.pub-cache/
+.pub/
+/build/
+
+# Symbolication related
+app.*.symbols
+
+# Obfuscation related
+app.*.map.json
+
+# Android Studio will place build artifacts here
+/android/app/debug
+/android/app/profile
+/android/app/release

web_embedding/element_embedding_demo/README.md

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+# element_embedding_demo
+
+This package contains the application used to demonstrate the
+upcoming Flutter web feature: "Element Embedding".
+
+This was first shown on the Flutter Forward event in Nairobi (Kenya), by Tim
+Sneath. [See the replay here](https://www.youtube.com/watch?v=zKQYGKAe5W8&t=5799s).
+
+## Running the demo
+
+The demo is a Flutter web app, so it can be run as:
+
+```terminal
+$ flutter run -d chrome
+```
+
+## Points of Interest
+
+* Check the new JS Interop:
+  * Look at `lib/main.dart`, find the `@js.JSExport()` annotation.
+  * Find the JS code that interacts with Dart in `web/js/demo-js-interop.js`.
+* See how the Flutter web application is embedded into the page now:
+  * Find `hostElement` in `web/index.html`.
+
+_(Built by @ditman, @kevmoo and @malloc-error)_