Upgrade SDK dependency to 2.12.0-0 (#107)

* Upgrade SDK dependency to 2.12.0-0


Co-authored-by: Nate Bosch <nbosch@google.com>
diff --git a/pubspec.yaml b/pubspec.yaml
index 0961e1a..a9d0b0e 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,14 +1,14 @@
 name: crypto
-version: 2.2.0-nullsafety-dev
+version: 2.2.0-nullsafety.0
 description: Implementations of SHA, MD5, and HMAC cryptographic functions
 homepage: https://www.github.com/dart-lang/crypto
 
 environment:
-  sdk: '>=2.10.0-2.0.dev <2.11.0'
+  sdk: '>=2.12.0-0 <3.0.0'
 
 dependencies:
   collection: ^1.15.0-nullsafety
-  convert: ^2.2.0-nullsafety
+  convert: ^3.0.0-nullsafety
   typed_data: ^1.3.0-nullsafety
 
 dev_dependencies: