* 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.