update dart min. version to 2.6

example/pubspec.lock

@@ -75,7 +75,7 @@ packages:
       path: ".."
       relative: true
     source: path
-    version: "1.0.1"
+    version: "1.0.2"
   flutter_test:
     dependency: "direct dev"
     description: flutter
@@ -199,4 +199,4 @@ packages:
     source: hosted
     version: "3.5.0"
 sdks:
-  dart: ">=2.4.0 <3.0.0"
+  dart: ">=2.6.0 <3.0.0"

pubspec.yaml

@@ -4,7 +4,7 @@ version: 1.0.2
 homepage: https://github.com/OpenFlutter/flutter_screenutil
 
 environment:
-  sdk: ">=2.0.0-dev.68.0 <3.0.0"
+  sdk: ">=2.6.0 <3.0.0"
 
 dependencies:
   flutter:
@@ -19,7 +19,6 @@ dev_dependencies:
 
 # The following section is specific to Flutter.
 flutter:
-
   # To add assets to your package, add an assets section, like this:
   # assets:
   #  - images/a_dot_burr.jpeg
@@ -30,7 +29,6 @@ flutter:
   #
   # An image asset can refer to one or more resolution-specific "variants", see
   # https://flutter.io/assets-and-images/#resolution-aware.
-
   # To add custom fonts to your package, add a fonts section here,
   # in this "flutter" section. Each entry in this list should have a
   # "family" key with the font family name, and a "fonts" key with a