THRIFT-3981 Enable analyzer strong mode in Dart library
Client: Dart
Patch: Mark Erickson <merickson@apache.org>

This closes #1135
diff --git a/tutorial/dart/client/pubspec.yaml b/tutorial/dart/client/pubspec.yaml
index 4bd3394..db64afc 100644
--- a/tutorial/dart/client/pubspec.yaml
+++ b/tutorial/dart/client/pubspec.yaml
@@ -22,7 +22,7 @@
 homepage: http://thrift.apache.org
 
 environment:
-  sdk: ^1.12.0
+  sdk: ">=1.13.0 <2.0.0"
 
 dependencies:
   browser: ^0.10.0