Migrate warnings.dart (#2787)

Author: srawlins

lib/src/dartdoc_options.dart

@@ -450,7 +450,7 @@ abstract class DartdocOption<T> {
 
   /// To avoid accessing early, call [add] on the option's parent before
   /// looking up unless this is a [DartdocRootOption].
-  late final DartdocOption parent;
+  late final DartdocOption<dynamic> parent;
 
   /// The [DartdocOptionRoot] containing this object.
   DartdocOptionRoot get root {
@@ -516,10 +516,13 @@ abstract class DartdocOption<T> {
   void addAll(Iterable<DartdocOption> options) => options.forEach(add);
 
   /// Get the immediate child of this node named [name].
-  DartdocOption<dynamic> operator [](String name) {
+  DartdocOption operator [](String name) {
     return _children[name]!;
   }
 
+  /// Get the immediate child of this node named [name] as a [DartdocOption<U>].
+  DartdocOption<U> getAs<U>(String name) => _children[name] as DartdocOption<U>;
+
   /// Apply the function [visit] to [this] and all children.
   void traverse(void Function(DartdocOption option) visit) {
     visit(this);

lib/src/model/accessor.dart

@@ -100,10 +100,12 @@ class Accessor extends ModelElement implements EnclosedElement {
   }
 
   @override
-  void warn(PackageWarning kind,
-      {String message,
-      Iterable<Locatable> referredFrom,
-      Iterable<String> extendedDebug}) {
+  void warn(
+    PackageWarning kind, {
+    String message,
+    Iterable<Locatable> referredFrom = const [],
+    Iterable<String> extendedDebug = const [],
+  }) {
     enclosingCombo.warn(kind,
         message: message,
         referredFrom: referredFrom,