Use stable SDK and dev deps (#544)

Author: natebosch

pubspec.yaml

@@ -4,7 +4,7 @@ homepage: https://github.com/dart-lang/http
 description: A composable, multi-platform, Future-based API for HTTP requests.
 
 environment:
-  sdk: '>=2.12.0-0 <3.0.0'
+  sdk: '>=2.12.0 <3.0.0'
 
 dependencies:
   http_parser: ^4.0.0
@@ -13,5 +13,5 @@ dependencies:
   pedantic: ^1.10.0
 
 dev_dependencies:
-  shelf: ^1.0.0-nullsafety.0
-  test: ^1.16.0-nullsafety.4
+  shelf: ^1.1.0
+  test: ^1.16.0