Update SDK constraint to allow 2.0.0-dev release (#7)

Corresponds to the constant renames JSON->json, etc
diff --git a/pubspec.yaml b/pubspec.yaml
index e8acb2f..5aedd11 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -6,4 +6,4 @@
 dev_dependencies:
   test: "^0.12.18"
 environment:
-  sdk: ">=1.99.0 <3.0.0"
+  sdk: ">=2.0.0-dev.8.0 <3.0.0"