* Fix some of the generic types specified to avoid runtime errors in
--preview-dart-2
type 'YamlMap' is not a subtype of type 'Map<String, dynamic>' where
YamlMap is from package:yaml/src/yaml_node.dart
Map is from dart:core
String is from dart:core
package:flutter_devicelab/framework/manifest.dart 17:36 loadTaskManifest
test/manifest_test.dart 14:33 main.<fn>.<fn>
package:test/src/backend/declarer.dart 161:27 Declarer.test.<fn>.<fn>.<fn>
* Add dynamic generic types to keep the analyer happy.
* Address code review comment.
