chore: adopt Dart 3.9 toolchain — honest floor + tall-style reformat (T-353)

Raise the declared minimums in pubspec.yaml to what our deps already
require: Flutter >=3.35.0 / Dart >=3.9.0 (was 3.19.0 / 3.5.0). alchemist
0.12 needs Flutter 3.32; Dart 3.9 first ships in Flutter 3.35, so 3.35 is
the binding floor. Pin the exact build toolchain in .fvmrc (Flutter
3.44.1).

Moving to the Dart 3.9 language level switches `dart format` to the new
"tall" style and enables two new lints. This commit is the resulting
mechanical churn, isolated from any behaviour change:
  - whole-tree `dart format` reformat (tall style)
  - `dart fix` for unnecessary_underscores + use_null_aware_elements

No runtime behaviour change; `make test` green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/files/ignore_test.dart

@@ -69,10 +69,7 @@ void main() {
 
     test('** crosses directory boundaries', () {
       final s = IgnoreSet.parse(const ['docs/**/*.draft.md\n']);
-      expect(
-        s.isIgnored('docs/deep/nested/a.draft.md', isDirectory: false),
-        isTrue,
-      );
+      expect(s.isIgnored('docs/deep/nested/a.draft.md', isDirectory: false), isTrue);
       expect(s.isIgnored('docs/a.draft.md', isDirectory: false), isTrue);
       expect(s.isIgnored('other/a.draft.md', isDirectory: false), isFalse);
     });

test/files/path_safety_test.dart

@@ -104,10 +104,7 @@ void main() {
       final link = Link('${root.path}/leak')..createSync(secret.path);
       expect(link.existsSync(), isTrue);
 
-      expect(
-        () => resolveUnderRootFollowingSymlinks(root, 'leak'),
-        throwsA(isA<PathOutsideRoot>()),
-      );
+      expect(() => resolveUnderRootFollowingSymlinks(root, 'leak'), throwsA(isA<PathOutsideRoot>()));
     });
 
     test('tolerates symlinks in the workspace root path itself', () {
@@ -130,10 +127,7 @@ void main() {
       Link('${root.path}/b').createSync(secret.path);
       Link('${root.path}/a').createSync('${root.path}/b');
 
-      expect(
-        () => resolveUnderRootFollowingSymlinks(root, 'a'),
-        throwsA(isA<PathOutsideRoot>()),
-      );
+      expect(() => resolveUnderRootFollowingSymlinks(root, 'a'), throwsA(isA<PathOutsideRoot>()));
     });
   });
 
@@ -171,10 +165,7 @@ void main() {
       addTearDown(() => outside.existsSync() ? outside.deleteSync(recursive: true) : null);
       File('${outside.path}/secret.txt').writeAsStringSync('payload');
       Link('${extra.path}/leak').createSync('${outside.path}/secret.txt');
-      expect(
-        () => resolveUnderRootsFollowingSymlinks(root, [extra], '${extra.absolute.path}/leak'),
-        throwsA(isA<PathOutsideRoot>()),
-      );
+      expect(() => resolveUnderRootsFollowingSymlinks(root, [extra], '${extra.absolute.path}/leak'), throwsA(isA<PathOutsideRoot>()));
     });
   });
 }

test/files/walk_test.dart

@@ -28,13 +28,7 @@ void main() {
   test('walks recursively, returns files sorted by path', () async {
     final r = await walkFiles(root: root, ignore: IgnoreSet([]));
     expect(r.truncated, isFalse);
-    expect(r.files.map((e) => e.path).toList(), [
-      'README.md',
-      'build/output.bin',
-      'lib/main.dart',
-      'lib/src/util.dart',
-      'pubspec.yaml',
-    ]);
+    expect(r.files.map((e) => e.path).toList(), ['README.md', 'build/output.bin', 'lib/main.dart', 'lib/src/util.dart', 'pubspec.yaml']);
   });
 
   test('prunes ignored directories (build/ via builtin ignore)', () async {

test/files/watcher_test.dart

@@ -15,12 +15,12 @@ void main() {
     test('fromEvent maps every FileSystemEvent.type to a wire kind', () {
       final dir = Directory.systemTemp;
       FileSystemEvent ev(int type) => switch (type) {
-            FileSystemEvent.create => FileSystemCreateEvent('${dir.path}/f', false),
-            FileSystemEvent.delete => FileSystemDeleteEvent('${dir.path}/f', false),
-            FileSystemEvent.modify => FileSystemModifyEvent('${dir.path}/f', false, false),
-            FileSystemEvent.move => FileSystemMoveEvent('${dir.path}/f', false, '${dir.path}/g'),
-            _ => FileSystemModifyEvent('${dir.path}/f', false, false),
-          };
+        FileSystemEvent.create => FileSystemCreateEvent('${dir.path}/f', false),
+        FileSystemEvent.delete => FileSystemDeleteEvent('${dir.path}/f', false),
+        FileSystemEvent.modify => FileSystemModifyEvent('${dir.path}/f', false, false),
+        FileSystemEvent.move => FileSystemMoveEvent('${dir.path}/f', false, '${dir.path}/g'),
+        _ => FileSystemModifyEvent('${dir.path}/f', false, false),
+      };
       expect(FileChangeKind.fromEvent(ev(FileSystemEvent.create)), FileChangeKind.created);
       expect(FileChangeKind.fromEvent(ev(FileSystemEvent.delete)), FileChangeKind.deleted);
       expect(FileChangeKind.fromEvent(ev(FileSystemEvent.modify)), FileChangeKind.modified);
@@ -65,15 +65,9 @@ void main() {
       // amount. firstWhere completes on the first matching event;
       // the timeout fails the test with a clear message if inotify
       // never delivers (instead of asserting on an empty list).
-      final saw = watcher.stream.firstWhere(
-        (c) => c.path == 'new.txt',
-        orElse: () => throw StateError('stream closed before new.txt arrived'),
-      );
+      final saw = watcher.stream.firstWhere((c) => c.path == 'new.txt', orElse: () => throw StateError('stream closed before new.txt arrived'));
       await File('${sandbox.path}/new.txt').writeAsString('hi');
-      final change = await saw.timeout(
-        ioTimeout,
-        onTimeout: () => fail('no `new.txt` event within ${ioTimeout.inSeconds}s'),
-      );
+      final change = await saw.timeout(ioTimeout, onTimeout: () => fail('no `new.txt` event within ${ioTimeout.inSeconds}s'));
       expect(change.path, 'new.txt');
     });