Extract large-dependency tests from model_test (#2068)

* dartfmt + comment

* fix up async memoization

* Change import for split model.dart

Author: jcollins-g

pubspec.yaml

@@ -29,6 +29,7 @@ dependencies:
   yaml: ^2.1.0
 
 dev_dependencies:
+  async: '>=2.0.8'
   build: ^1.0.1
   build_runner: ^1.0.0
   build_version: ^2.0.0