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].

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[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]:
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Co-authored-by: Mark Thompson
<2554588+MarkTechson@users.noreply.github.com>
Co-authored-by: David Iglesias <ditman@gmail.com>

Co-authored-by: Mark Thompson <2554588+MarkTechson@users.noreply.github.com>
Co-authored-by: David Iglesias <ditman@gmail.com>

web_embedding/element_embedding_demo/web/js/demo-js-interop.js

@@ -0,0 +1,43 @@
+// Sets up a channel to JS-interop with Flutter
+(function() {
+  "use strict";
+  // This function will be called from Flutter when it prepares the JS-interop.
+  window._stateSet = function () {
+    window._stateSet = function () {
+      console.log("Call _stateSet only once!");
+    };
+
+    // The state of the flutter app, see `class _MyAppState` in lib/main.dart.
+    let appState = window._appState;
+
+    let valueField = document.querySelector("#value");
+    let updateState = function () {
+      valueField.value = appState.count;
+    };
+
+    // Register a callback to update the HTML field from Flutter.
+    appState.addHandler(updateState);
+
+    // Render the first value (0).
+    updateState();
+
+    let incrementButton = document.querySelector("#increment");
+    incrementButton.addEventListener("click", (event) => {
+      appState.increment();
+    });
+
+    let screenSelector = document.querySelector("#screen-selector");
+    screenSelector.addEventListener("change", (event) => {
+      appState.changeDemoScreenTo(event.target.value);
+      setJsInteropControlsEnabled(event.target.value === 'counter');
+    });
+
+    // Enables/disables the Value/Increment controls.
+    function setJsInteropControlsEnabled(enabled) {
+      let elements = document.querySelectorAll("#increment, label[for='value']");
+      elements.forEach((el) => {
+        el.classList.toggle('disabled', !enabled);
+      })
+    }
+  };
+}());