implicit-casts: false still ignored

analysis_options.yaml

# Supported lint rules and documentation:
#   http://dart-lang.github.io/linter/lints/

analyzer:
  exclude: [build/**]
  strong-mode:
    implicit-casts: false
    implicit-dynamic: false

linter:
  rules:
    #- always_declare_return_types
    #- always_specify_types
    - annotate_overrides
    #- avoid_as
    - avoid_empty_else
    - avoid_init_to_null
    - avoid_relative_lib_imports
    - avoid_return_types_on_setters
    - avoid_types_as_parameter_names
    - await_only_futures
    - camel_case_types
    - cancel_subscriptions
    - close_sinks
    - comment_references
    - constant_identifier_names
    - control_flow_in_finally
    - empty_catches
    - empty_constructor_bodies
    - empty_statements
    - hash_and_equals
    - implementation_imports
    - iterable_contains_unrelated_type
    - library_names
    - library_prefixes
    - list_remove_unrelated_type
    - no_duplicate_case_values
    - non_constant_identifier_names
    - one_member_abstracts
    - only_throw_errors
    - overridden_fields
    - package_api_docs
    - package_names
    - package_prefixed_library_names
    - prefer_contains
    - prefer_equal_for_default_values
    - prefer_is_not_empty
    #- public_member_api_docs
    - recursive_getters
    - slash_for_doc_comments
    #- sort_constructors_first # TODO: reinstate after https://github.com/dart-lang/site-www/issues/1196 is resolved?
    - sort_unnamed_constructors_first
    - super_goes_last
    - test_types_in_equals
    - throw_in_finally
    - type_annotate_public_apis
    - type_init_formals
    #- unawaited_futures
    - unnecessary_brace_in_string_interps
    - unnecessary_const
    - unnecessary_getters_setters
    - unnecessary_new
    - unrelated_type_equality_checks
    - use_rethrow_when_possible
    - valid_regexps

main.dart

void showImplicitDynamic(Object someValue) {
  // #docregion implicit-dynamic
  Object o = someValue;
  String s = o; // Implicit downcast
  String s2 = s.substring(1);
  // #enddocregion implicit-dynamic
  print('substring: $s2');
}

void main() {
  showImplicitDynamic(Object());
//   showImplicitDynamic('abc');
}

pubspec.yaml

name: analysis_options_samples
description: Example code for dartlang.org/guides/language/analysis-options.

environment:
  sdk: '>=2.1.0 <3.0.0'

# dev_dependencies:
  # dartlang_examples_util:
    # path: ../util