linter: unreachable_from_main: consider any 'toJson' as reachable

Fixes https://github.com/dart-lang/linter/issues/4495

Change-Id: Ic6ba67393f9c05d8fa1b327b8d2c87f654fc01d0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/362561
Auto-Submit: Samuel Rawlins <[email protected]>
Commit-Queue: Samuel Rawlins <[email protected]>
Reviewed-by: Nate Bosch <[email protected]>

Author: srawlins

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

@@ -280,6 +280,20 @@ class _ReferenceVisitor extends RecursiveAstVisitor {
     super.visitExtensionTypeDeclaration(node);
   }
 
+  @override
+  void visitMethodDeclaration(MethodDeclaration node) {
+    if (node.name.lexeme == 'toJson' && !node.isStatic) {
+      // The 'dart:convert' library uses dynamic invocation to call `toJson` on
+      // arbitrary objects. Any declaration of `toJson` is automatically
+      // reachable.
+      var element = node.declaredElement;
+      if (element != null) {
+        _addDeclaration(element);
+      }
+    }
+    super.visitMethodDeclaration(node);
+  }
+
   @override
   void visitNamedType(NamedType node) {
     var element = node.element;

pkg/linter/test/rules/unreachable_from_main_test.dart

@@ -1051,6 +1051,20 @@ extension E on int {
     ]);
   }
 
+  test_instanceMethod_reachable_toJson() async {
+    await assertNoDiagnostics(r'''
+import 'dart:convert';
+
+void main() async {
+  jsonEncode([C()]);
+}
+
+class C {
+  List<Object> toJson() => ['c'];
+}
+''');
+  }
+
   test_instanceMethod_unreachable_inExtensionType() async {
     await assertDiagnostics(r'''
 void main() {