Adds "Element Embedding" demo package. (#1596)

experimental/element_embedding_demo/pubspec.yaml

@@ -0,0 +1,23 @@
+name: element_embedding_demo
+description: A small app to be embedded into a HTML element (see web/index.html)
+publish_to: 'none'
+version: 1.0.0+1
+
+environment:
+  sdk: '>=3.0.0-0 <4.0.0'
+
+dependencies:
+  cupertino_icons: ^1.0.2
+  flutter:
+    sdk: flutter
+  js: ^0.6.6
+
+dev_dependencies:
+  flutter_test:
+    sdk: flutter
+  flutter_lints: ^2.0.0
+
+flutter:
+  uses-material-design: true
+  assets:
+    - assets/dash.png