Remove assertion that is no longer valid.

I actually can't imagine how it was ever valid. The
`InheritingContainer.inheritanceChain` implementations can always result in a
list that contains two instances of Object.

Also make `inheritance` private, and improve some related tests.

Author: srawlins

lib/src/generator/templates.runtime_renderers.dart

@@ -6938,19 +6938,6 @@ class _Renderer_Inheritable extends RendererBase<Inheritable> {
                         parent: r);
                   },
                 ),
-                'inheritance': Property(
-                  getValue: (CT_ c) => c.inheritance,
-                  renderVariable: (CT_ c, Property<CT_> self,
-                          List<String> remainingNames) =>
-                      self.renderSimpleVariable(
-                          c, remainingNames, 'List<InheritingContainer>'),
-                  renderIterable: (CT_ c, RendererBase<CT_> r,
-                      List<MustachioNode> ast, StringSink sink) {
-                    return c.inheritance.map((e) => _render_InheritingContainer(
-                        e, ast, r.template, sink,
-                        parent: r));
-                  },
-                ),
                 'isCovariant': Property(
                   getValue: (CT_ c) => c.isCovariant,
                   renderVariable: (CT_ c, Property<CT_> self,
@@ -16216,7 +16203,6 @@ const _invisibleGetters = {
     'attributes',
     'canonicalLibrary',
     'canonicalModelElement',
-    'inheritance',
     'isCovariant',
     'isInherited',
     'isOverride',

lib/src/model/inheritable.dart

@@ -58,7 +58,7 @@ mixin Inheritable on ContainerMember {
       Container? previous;
       Container? previousNonSkippable;
       Container? found;
-      for (var c in inheritance.reversed) {
+      for (var c in _inheritance.reversed) {
         // Filter out mixins.
         if (c.containsElement(searchElement)) {
           if ((packageGraph.inheritThrough.contains(previous) &&
@@ -96,7 +96,11 @@ mixin Inheritable on ContainerMember {
     return super.computeCanonicalEnclosingContainer();
   }
 
-  List<InheritingContainer> get inheritance {
+  /// A roughly ordered list of this element's enclosing container's inheritance
+  /// chain.
+  ///
+  /// See [InheritingContainer.inheritanceChain] for details.
+  List<InheritingContainer> get _inheritance {
     var inheritance = [
       ...(enclosingElement as InheritingContainer).inheritanceChain,
     ];
@@ -120,7 +124,6 @@ mixin Inheritable on ContainerMember {
     if (inheritance.last != object) {
       inheritance.add(object);
     }
-    assert(inheritance.where((e) => e == object).length == 1);
     return inheritance;
   }
 

test/classes_test.dart

@@ -2,6 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+import 'package:dartdoc/src/special_elements.dart';
 import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
@@ -182,5 +183,21 @@ class C<T> implements A<T>, _B<T> {}
     expect(c.publicInterfaces.first.modelElement, library.classes.named('A'));
   }
 
+  void test_inheritanceOfObjectInstanceMethod() async {
+    // This code is written such that `Inheritable._inheritance` for
+    // `A.toString()` includes two copies of Object; one from walking up B's
+    // chain, and then the second from walking up C's chain.
+    var library = await bootPackageWithLibrary('''
+class A implements B, C {}
+class B implements C {}
+class C implements D {}
+class D implements Object {}
+''');
+
+    var object = library.packageGraph.specialClasses[SpecialClass.object]!;
+    var toString = library.classes.named('A').instanceMethods.named('toString');
+    expect(toString.canonicalEnclosingContainer, object);
+  }
+
   // TODO(srawlins): Test everything else about classes.
 }

test/end2end/model_special_cases_test.dart

@@ -10,6 +10,7 @@
 library;
 
 import 'package:async/async.dart';
+import 'package:dartdoc/src/element_type.dart';
 import 'package:dartdoc/src/matching_link_result.dart';
 import 'package:dartdoc/src/model/model.dart';
 import 'package:dartdoc/src/model_utils.dart';
@@ -413,14 +414,21 @@ void main() {
           .singleWhere((f) => f.name == 'hashCode');
       var objectModelElement =
           sdkAsPackageGraph.specialClasses[SpecialClass.object];
-      // If this fails, EventTarget might have been changed to no longer
-      // inherit from Interceptor.  If that's true, adjust test case to
-      // another class that does.
-      expect(hashCode.inheritance.any((c) => c.name == 'Interceptor'), isTrue);
-      // If EventTarget really does start implementing hashCode, this will
-      // fail.
-      expect(hashCode.href,
-          equals('${htmlBasePlaceholder}dart-core/Object/hashCode.html'));
+      expect(
+        eventTarget.superChain,
+        contains(isA<ParameterizedElementType>()
+            .having((t) => t.name, 'name', 'Interceptor')),
+        reason: 'EventTarget appears to no longer subtype Interceptor, which '
+            'makes the premise of this test invalid. To keep the test case '
+            'valid, we need to use a class that subclasses Interceptor.',
+      );
+      expect(
+        hashCode.href,
+        equals('${htmlBasePlaceholder}dart-core/Object/hashCode.html'),
+        reason:
+            "EventTarget appears to have an explicit override of 'hashCode', "
+            'which makes this test case invalid.',
+      );
       expect(hashCode.canonicalEnclosingContainer, equals(objectModelElement));
       expect(
           eventTarget.publicSuperChainReversed