Update to recommended Dart 2 constraint sdk: '>=2.0.0-dev.61.0 <3.0.0'

Author: mythz

README.md

@@ -29,7 +29,7 @@ To make API calls we need to use the `JsonServiceClient`, installed by adding th
 
 ```yaml
 dependencies:
-  servicestack: "^1.0.2"
+  servicestack: ^1.0.3
 ```
 
 Saving `pubspec.yaml` in VS Code with the [Dart Code Extension](https://dartcode.org) automatically calls `pub get` or `flutter packages get` (in Flutter projects) to add any new dependencies to your project.
@@ -181,8 +181,10 @@ This creates a basic Flutter App which you can run in your Android Device or And
 
 Then to use `JsonServiceClient` add the `servicestack` dependency to your apps [pubspec.yaml](https://github.com/ServiceStackApps/HelloFlutter/blob/master/pubspec.yaml):
 
-  dependencies:
-    servicestack: "^1.0.2"
+```yaml
+dependencies:
+  servicestack: ^1.0.3
+```
 
 Saving `pubspec.yaml` automatically runs [flutter packages get](https://flutter.io/using-packages/) to install any new dependencies in your App. 
 

pubspec.yaml

@@ -1,10 +1,10 @@
 name: servicestack
-version: 1.0.2
+version: 1.0.3
 author: ServiceStack, Inc <team@servicestack.net>
 description: ServiceStack's convenience utils for developing Dart VM and flutter apps. Integrates with ServiceStack's Server features including ServiceClient, Error Handling and Validation
 homepage: https://github.com/ServiceStack/servicestack-dart
 environment:
-  sdk: '>=2.0.0 <3.0.0'
+  sdk: '>=2.0.0-dev.61.0 <3.0.0'
 dependencies:
   http: "^0.11.3+16"
 dev_dependencies: