[shared_preferences] Update macOS for NNBD

macOS federated plugin implementations that contain no Dart code just
need their Dart SDK bumped in order to be considered nullsafe.

Author: stuartmorgan-g

packages/shared_preferences/shared_preferences_macos/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 0.0.2-nullsafety
+
+* Update Dart SDK constraint for null safety.
+
 ## 0.0.1+12
 
 * Update Flutter SDK constraint.

packages/shared_preferences/shared_preferences_macos/pubspec.yaml

@@ -3,7 +3,7 @@ description: macOS implementation of the shared_preferences plugin.
 # 0.0.y+z is compatible with 1.0.0, if you land a breaking change bump
 # the version to 2.0.0.
 # See more details: https://github.com/flutter/flutter/wiki/Package-migration-to-1.0.0
-version: 0.0.1+12
+version: 0.0.2-nullsafety
 homepage: https://github.com/flutter/plugins/tree/master/packages/shared_preferences/shared_preferences_macos
 
 flutter:
@@ -13,7 +13,7 @@ flutter:
         pluginClass: SharedPreferencesPlugin
 
 environment:
-  sdk: ">=2.1.0 <3.0.0"
+  sdk: ">=2.12.0-0 <3.0.0"
   flutter: ">=1.12.8"
 
 dependencies: