Add workflow to deploy the sample index (#791)

* Add workflow to build and deploy the sample index

* update gh-pages action

* fix yaml

* create web/ directory in build

* grammar

* add ignored directories

* revert pubspec.lock files

* add job to run _tool/verify_samples.dart

* Update filipino_cuisine for Flutter 2

* remove timeflow demo.

The unnamed List constructor is now deprecated, refactoring
this code to use add() requires more knowledge about the code for
this demo.
https://dart.dev/null-safety/understanding-null-safety#no-unnamed-list-constructor

* update slide_puzzle

* ensure stable channel is used to verify

* move verify web demos action into separate yaml file - avoid running
on each flutter version.

* add on: pull_request

* update slide_puzzle

* Update gh-pages.yml

* Add copyright header

.github/workflows/verify-web-demos.yml

@@ -0,0 +1,20 @@
+name: Verify web demos
+on: [push, pull_request]
+jobs:
+  verify-web-demos:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+        with:
+          submodules: true
+          fetch-depth: 0
+      - uses: subosito/flutter-action@v1
+        with:
+          channel: stable
+      - name: Init scripts
+        run: dart pub get
+        working-directory: web/_tool
+      - name: Verify packages
+        run: dart _tool/verify_packages.dart
+        working-directory: web