linter: catch fields referenced in a pattern field

Fixes https://github.com/dart-lang/sdk/issues/53674

Change-Id: I87effd3cc4b3d3227cd26c3fdeb68b7f7e2d0ca9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/330880
Reviewed-by: Phil Quitslund <pquitslund@google.com>
Commit-Queue: Phil Quitslund <pquitslund@google.com>
Auto-Submit: Samuel Rawlins <srawlins@google.com>

pkg/linter/lib/src/rules/unreachable_from_main.dart

@@ -315,6 +315,15 @@ class _ReferenceVisitor extends RecursiveAstVisitor {
     }
   }
 
+  @override
+  void visitPatternField(PatternField node) {
+    var e = node.element;
+    if (e != null) {
+      _addDeclaration(e);
+    }
+    super.visitPatternField(node);
+  }
+
   @override
   void visitPostfixExpression(PostfixExpression node) {
     _visitCompoundAssignmentExpression(node);

pkg/linter/test/rules/unreachable_from_main_test.dart

@@ -20,6 +20,22 @@ class UnreachableFromMainTest extends LintRuleTest {
   @override
   String get lintRule => 'unreachable_from_main';
 
+  test_class_instanceField_reachable_matchedInPattern() async {
+    await assertNoDiagnostics(r'''
+void main() {
+  var x = switch (C(1)) {
+    C(:final f) => f.round(),
+  };
+  print(x);
+}
+
+class C {
+  final int f;
+  C(this.f);
+}
+''');
+  }
+
   test_class_instanceField_reachable_overrides_local() async {
     await assertDiagnostics(r'''
 void main() {