Upgrade to Dart 2.0

Author: mythz

.vscode/launch.json

@@ -9,7 +9,6 @@
             "name": "Dart",
             "program": "${workspaceFolder}/test/client_test.dart",
             "args": [
-                "--preview-dart-2"
             ],
             "request": "launch",
             "type": "dart"

pubspec.yaml

@@ -4,8 +4,9 @@ author: ServiceStack, Inc <[email protected]>
 description: ServiceStack's Dart library providing convenience utilities in developing Dart and flutter apps. Integrates with ServiceStack's Server features including ServiceClient, Error Handling and Validation
 homepage: https://github.com/ServiceStack/servicestack-dart
 environment:
-  sdk: ">=1.19.0 <2.0.0"
+  sdk: '>=2.0.0-dev.48 <3.0.0'
 dependencies:
   http: "^0.11.3+16"
 dev_dependencies:
-  test: "^0.12.33"
+  flutter_test:
+    sdk: flutter