enable and fix a new lint (#809)

Author: kevmoo

analysis_options.yaml

@@ -8,6 +8,7 @@ linter:
     - always_require_non_null_named_parameters
     - annotate_overrides
     - avoid_catching_errors
+    - avoid_dynamic_calls
     - avoid_empty_else
     - avoid_function_literals_in_foreach_calls
     - avoid_init_to_null

json_serializable/tool/doc_builder.dart

@@ -21,7 +21,7 @@ class _DocBuilder extends Builder {
     final lockFileAssetId = AssetId(buildStep.inputId.package, 'pubspec.lock');
     final lockFileContent = await buildStep.readAsString(lockFileAssetId);
     final lockFileYaml =
-        loadYaml(lockFileContent, sourceUrl: lockFileAssetId.uri);
+        loadYaml(lockFileContent, sourceUrl: lockFileAssetId.uri) as YamlMap;
     final pkgMap = lockFileYaml['packages'] as YamlMap;
     final jsonAnnotationMap = pkgMap['json_annotation'] as YamlMap;
     final jsonAnnotationVersionString = jsonAnnotationMap['version'] as String;