Prepare for NON_NULLABLE_EQUALS_PARAMETER diagnostic (#4523)

This prepares for the new
[`NON_NULLABLE_EQUALS_PARAMETER`](osa1/sdk@48ee1f2)
diagnostic discussed in https://github.com/dart-lang/linter/issues/3441.

Author: parlough

examples/misc/lib/effective_dart/design_bad.dart

@@ -1,6 +1,7 @@
 // ignore_for_file: close_sinks, type_annotate_public_apis, unused_element, unused_local_variable, avoid_types_as_parameter_names
 // ignore_for_file: type_init_formals, unused_field, always_declare_return_types, strict_raw_type, prefer_typing_uninitialized_variables
 // ignore_for_file: use_function_type_syntax_for_parameters, prefer_generic_function_type_aliases, avoid_null_checks_in_equality_operators
+// ignore_for_file: non_nullable_equals_parameter
 
 import 'dart:async';
 

examples/misc/lib/library_tour/core/hash_code.dart

@@ -13,7 +13,7 @@ class Person {
   // You should generally implement operator `==` if you
   // override `hashCode`.
   @override
-  bool operator ==(dynamic other) {
+  bool operator ==(Object other) {
     return other is Person &&
         other.firstName == firstName &&
         other.lastName == lastName;

src/_guides/libraries/library-tour.md

@@ -958,7 +958,7 @@ class Person {
   // You should generally implement operator `==` if you
   // override `hashCode`.
   @override
-  bool operator ==(dynamic other) {
+  bool operator ==(Object other) {
     return other is Person &&
         other.firstName == firstName &&
         other.lastName == lastName;