Support SDK v2-dev (#38)

diff --git a/pubspec.yaml b/pubspec.yaml
index 81ea3c6..4d71cb1 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -4,7 +4,7 @@
 description: Library of cryptographic functions.
 homepage: https://www.github.com/dart-lang/crypto
 environment:
-  sdk: '>=1.16.0 <2.0.0'
+  sdk: '>=1.16.0 <2.0.0-dev.infinity'
 dependencies:
   collection: '^1.0.0'
   convert: '>=1.0.0 <3.0.0'