Update Dart2 sample to Dart 2.17 (#7482)

Use *new* official Dart Docker image
Migrate to newly recommended lints

frameworks/Dart/dart2/analysis_options.yaml

@@ -1,7 +1,8 @@
-include: package:pedantic/analysis_options.yaml
+include: package:lints/recommended.yaml
 analyzer:
-  strong-mode:
-    implicit-casts: false
+  language:
+    strict-casts: true
+
 linter:
   rules:
     - avoid_dynamic_calls

frameworks/Dart/dart2/dart2.dockerfile

@@ -1,10 +1,10 @@
-FROM google/dart:2.12
+FROM dart:2.17.6
 
 WORKDIR /dart_app
 COPY pubspec.yaml pubspec.yaml
 COPY server.dart server.dart
 
-RUN pub upgrade
+RUN dart pub upgrade
 
 EXPOSE 8080
 

frameworks/Dart/dart2/pubspec.yaml

@@ -1,7 +1,7 @@
 name: dartbenchmark
 description: A benchmark of dart
 environment:
-  sdk: '>=2.12.0 <3.0.0'
+  sdk: '>=2.17.0 <3.0.0'
 
 dev_dependencies:
-  pedantic: ^1.0.0
+  lints: ^2.0.0