Update pubspec to require Dart 1.25.
diff --git a/pubspec.yaml b/pubspec.yaml
index 94054ec..b99f7d1 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -17,4 +17,4 @@
 dev_dependencies:
   test: '^0.12.17'
 environment:
-  sdk: ">=1.21.0 <2.0.0"
+  sdk: ">=1.25.0 <2.0.0"