Update required version number.
diff --git a/pubspec.yaml b/pubspec.yaml
index 65f392e..cc461f8 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -27,7 +27,7 @@
   test: '^0.12.18+1'
   stack_trace: '>=0.9.1 <2.0.0'
 environment:
-  sdk: ">=1.25.0 <2.0.0"
+  sdk: ">=2.0.0-dev <3.0.0"
 transformers:
 - observe:
     files: