lint: Suppress unnecessary_cast in generated files

For how we found out how to do this, see:
  google/json_serializable.dart#625

Author: chrisbobbe

build.yaml

@@ -0,0 +1,8 @@
+targets:
+  $default:
+    builders:
+      source_gen:combining_builder:
+        options:
+          ignore_for_file:
+            # https://github.com/google/json_serializable.dart/issues/625
+            - unnecessary_cast