Update dart_style to pass in a language version to DartFormatter.

We're in [the process of](dart-lang/dart_style#1403) moving dart_style to [a new formatting style](dart-lang/dart_style#1253). That involves making the formatter [aware of the language version of what it's formatting](dart-lang/dart_style#1402).

That in turn means that the library API now lets you pass in a language version. In dart_style 2.3.7 [you can pass in a language version but the parameter is optional](https://pub.dev/documentation/dart_style/latest/dart_style/DartFormatter/DartFormatter.html). In the forthcoming 3.0.0 release, that parameter will become mandatory.

This updates every call to `DartFormatter()` to pass in the latest language version. If there's a more specific version that should be used, let me know and I'll update the PR.

Thanks!

README.md

@@ -25,7 +25,10 @@ void main() {
       ..name = 'eat'
       ..body = const Code("print('Yum!');"))));
   final emitter = DartEmitter();
-  print(DartFormatter().format('${animal.accept(emitter)}'));
+  print(
+    DartFormatter(languageVersion: DartFormatter.latestLanguageVersion)
+        .format('${animal.accept(emitter)}'),
+  );
 }
 ```
 
@@ -57,7 +60,10 @@ void main() {
           ..returns = refer('Other', 'package:b/b.dart')),
       ]));
   final emitter = DartEmitter.scoped();
-  print(DartFormatter().format('${library.accept(emitter)}'));
+  print(
+    DartFormatter(languageVersion: DartFormatter.latestLanguageVersion)
+        .format('${library.accept(emitter)}'),
+  );
 }
 ```
 
@@ -103,7 +109,7 @@ run from the snapshot instead of from source to avoid problems with deleted
 files. These steps must be run without deleting the source files.
 
 ```bash
-./tool/regenerate.sh 
+./tool/regenerate.sh
 ```
 
 [build_runner]: https://pub.dev/packages/build_runner

example/example.dart

@@ -5,7 +5,9 @@
 import 'package:code_builder/code_builder.dart';
 import 'package:dart_style/dart_style.dart';
 
-final _dartfmt = DartFormatter();
+final _dartfmt = DartFormatter(
+  languageVersion: DartFormatter.latestLanguageVersion,
+);
 
 void main() {
   print('animalClass():\n${'=' * 40}\n${animalClass()}');

pubspec.yaml

@@ -18,6 +18,6 @@ dev_dependencies:
   build_runner: ^2.0.3
   built_value_generator: ^8.0.0
   dart_flutter_team_lints: ^3.0.0
-  dart_style: ^2.3.4
+  dart_style: ^2.3.7
   source_gen: ^1.0.0
   test: ^1.16.0

test/common.dart

@@ -5,12 +5,15 @@
 import 'package:code_builder/code_builder.dart';
 import 'package:dart_style/dart_style.dart';
 
-final DartFormatter _dartfmt = DartFormatter();
 String _format(String source) {
+  final formatter = DartFormatter(
+    languageVersion: DartFormatter.latestLanguageVersion,
+  );
+
   try {
-    return _dartfmt.format(source);
+    return formatter.format(source);
   } on FormatterException catch (_) {
-    return _dartfmt.formatStatement(source);
+    return formatter.formatStatement(source);
   }
 }