"Migrate" dummy packages to null safety (#84013)

dev/docs/platform_integration/pubspec.yaml

 name: platform_integration
 
+environment:
+  sdk: ">=2.12.0 <3.0.0"

packages/flutter_test/test/test_config/project_root/pubspec.yaml

@@ -4,6 +4,6 @@
 name: dummy
 
 environment:
-  sdk: '>=2.10.0 <3.0.0'
+  sdk: '>=2.12.0 <3.0.0'
 
 # PUBSPEC CHECKSUM: 0000

packages/flutter_tools/test/data/asset_test/font/pubspec.yaml

@@ -2,7 +2,7 @@ name: font
 description: A test project that contains a font.
 
 environment:
-  sdk: ">=2.0.0-dev.68.0 <3.0.0"
+  sdk: ">=2.12.0 <3.0.0"
 
 flutter:
   uses-material-design: true

packages/flutter_tools/test/data/asset_test/main/pubspec.yaml

@@ -2,7 +2,7 @@ name: main
 description: A test project that has a package with a font as a dependency.
 
 environment:
-  sdk: ">=2.0.0-dev.68.0 <3.0.0"
+  sdk: ">=2.12.0 <3.0.0"
 
 dependencies:
   font:

packages/integration_test/integration_test_macos/pubspec.yaml

@@ -11,7 +11,7 @@ flutter:
         pluginClass: IntegrationTestPlugin
 
 environment:
-  sdk: ">=2.1.0 <3.0.0"
+  sdk: ">=2.12.0 <3.0.0"
 
 dependencies:
   flutter: