Windows nullsafety prep (flutter#3442)

shared_preferences_windows depends on path_provider_windows, and both
use 'ffi'. This relaxes the 'ffi' version constraint on
shared_preferences_windows to allow path_provider_windows to be migrated
to null-safety (which requires updating to the nullsafe version of ffi).

Part of flutter#70229

Author: stuartmorgan-g

packages/shared_preferences/shared_preferences_windows/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 0.0.2+2
+
+* Relax 'ffi' version constraint.
+
 ## 0.0.2+1
 
 * Update Flutter SDK constraint.

packages/shared_preferences/shared_preferences_windows/pubspec.yaml

@@ -1,7 +1,7 @@
 name: shared_preferences_windows
 description: Windows implementation of shared_preferences
 homepage: https://github.com/flutter/plugins/tree/master/packages/shared_preferences/shared_preferences_windows
-version: 0.0.2+1
+version: 0.0.2+2
 
 flutter:
   plugin:
@@ -18,7 +18,7 @@ dependencies:
   shared_preferences_platform_interface: ^1.0.0
   flutter:
     sdk: flutter
-  ffi: ^0.1.3
+  ffi: ">=0.1.3 < 0.3.0"
   file: ">=5.1.0 <7.0.0"
   meta: ^1.1.7
   path: ^1.6.4