Split CI testing based on flutter channels (#651)

experimental/web_dashboard/lib/src/auth/firebase.dart

@@ -54,4 +54,3 @@ class _FirebaseUser implements User {
 
   _FirebaseUser(this.uid);
 }
-

experimental/web_dashboard/lib/src/auth/mock.dart

@@ -13,7 +13,7 @@ class MockAuthService implements Auth {
   Future<User> signIn() async {
     // Sign in will randomly fail 25% of the time.
     var random = Random();
-    if (random.nextInt(4) == 0)  {
+    if (random.nextInt(4) == 0) {
       throw SignInException();
     }
     return MockUser();

experimental/web_dashboard/lib/src/pages/sign_in.dart

@@ -95,7 +95,7 @@ class _SignInButtonState extends State<SignInButton> {
   }
 
   void _showError() {
-    Scaffold.of(context).showSnackBar(
+    ScaffoldMessenger.of(context).showSnackBar(
       SnackBar(
         content: Text('Unable to sign in.'),
       ),