Be slightly more lenient about the assertion, as it differs on different backends.

lib/web_ui/test/canvaskit/embedded_views_test.dart

@@ -417,7 +417,7 @@ void testMain() {
       } on AssertionError catch (error) {
         expect(
           error.toString(),
-          'Assertion failed: "Cannot render platform views: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15. These views have not been created, or they have been deleted."',
+          contains('Cannot render platform views: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15. These views have not been created, or they have been deleted.'),
         );
       }