Revert use of const asserts in flutter_driver (#10568)

Fixes bot breakage resulting from commit
7d71326363 (#10540).

packages/flutter_driver/lib/src/health.dart

@@ -29,8 +29,9 @@ final EnumIndex<HealthStatus> _healthStatusIndex =
 
 class Health extends Result {
   /// Creates a [Health] object with the given [status].
-  Health(this.status)
-    : assert(status != null);
+  Health(this.status) {
+    assert(status != null);
+  }
 
   /// Deserializes the result from JSON.
   static Health fromJson(Map<String, dynamic> json) {