Update minimal Dart version requirement to 2.5.0

Author: nalenz-objectbox

examples/flutter/objectbox_demo_desktop/pubspec.yaml

@@ -4,7 +4,7 @@ description: An example project for the objectbox-dart binding.
 version: 0.2.0
 
 environment:
-  sdk: '>=2.5.0-dev.2.0 <3.0.0'
+  sdk: '>=2.5.0 <3.0.0'
   flutter: '>=1.9.8-pre.31'
 
 dependencies:

objectbox/pubspec.yaml

@@ -3,7 +3,7 @@ version: 0.2.0
 description: >-
   ObjectBox binding for Dart.
 environment:
-  sdk: '>=2.5.0-dev.2.0 <3.0.0'
+  sdk: '>=2.5.0 <3.0.0'
 dependencies:
   flat_buffers: ^1.11.0
   utf: ^0.9.0

objectbox_model_generator/pubspec.yaml

@@ -3,7 +3,7 @@ version: 0.2.0
 description: >-
   A preprocessor for Dart source files containing ObjectBox entity definitions.
 environment:
-  sdk: '>=2.5.0-dev.2.0 <3.0.0'
+  sdk: '>=2.5.0 <3.0.0'
 dependencies:
   build: '>=0.12.0 <2.0.0'
   source_gen: ^0.9.0

objectbox_test/pubspec.yaml

@@ -3,7 +3,7 @@ version: 0.2.0
 description: >-
   A simple tester for the ObjectBox Dart binding.
 environment:
-  sdk: '>=2.5.0-dev.2.0 <3.0.0'
+  sdk: '>=2.5.0 <3.0.0'
 dependencies:
   objectbox:
     path: ../objectbox