Fix an enum-related assert in canonicalization (#2812)

* Allow magical accessors from nowhere if they are Enum related

* intermediate

* dartfmt

* Add a stub Enum class since that's now in the required list

Author: jcollins-g

lib/src/model/enum.dart

@@ -6,13 +6,13 @@
 import 'package:analyzer/dart/element/element.dart';
 import 'package:dartdoc/src/model/model.dart';
 import 'package:dartdoc/src/render/enum_field_renderer.dart';
+import 'package:dartdoc/src/special_elements.dart';
 
-class Enum extends InheritingContainer {
+class Enum extends InheritingContainer with TypeImplementing {
   Enum(ClassElement element, Library library, PackageGraph packageGraph)
       : super(element, library, packageGraph);
 
   List<InheritingContainer> _inheritanceChain;
-
   @override
   List<InheritingContainer> get inheritanceChain {
     if (_inheritanceChain == null) {
@@ -23,6 +23,12 @@ class Enum extends InheritingContainer {
           in superChain.map((e) => (e.modelElement as InheritingContainer))) {
         _inheritanceChain.addAll(c.inheritanceChain);
       }
+
+      _inheritanceChain.addAll(interfaces.expand(
+          (e) => (e.modelElement as InheritingContainer).inheritanceChain));
+
+      assert(_inheritanceChain
+          .contains(packageGraph.specialClasses[SpecialClass.enumClass]));
     }
     return _inheritanceChain.toList(growable: false);
   }

lib/src/special_elements.dart

@@ -23,6 +23,9 @@ enum SpecialClass {
 
   /// From dart:core, pragma
   pragma,
+
+  /// From dart:core, Enum
+  enumClass,
 }
 
 /// A declaration of a special [Class] and how to find it.
@@ -70,6 +73,8 @@ class _SpecialClassDefinition {
 const Map<String, _SpecialClassDefinition> _specialClassDefinitions = {
   'Object': _SpecialClassDefinition(
       SpecialClass.object, 'Object', 'dart.core', 'dart:core'),
+  'Enum': _SpecialClassDefinition(
+      SpecialClass.enumClass, 'Enum', 'dart.core', 'dart:core'),
   'Interceptor': _SpecialClassDefinition(SpecialClass.interceptor,
       'Interceptor', '_interceptors', 'dart:_interceptors',
       required: false),

testing/sky_engine/sdk/core.dart

@@ -57,6 +57,12 @@ abstract class Map<K, V> extends Object {
 
 class Null extends Object {}
 
+abstract class Enum {
+  int get index;
+
+  String get _name;
+}
+
 abstract class num implements Comparable<num> {
   num operator %(num other);
   num operator *(num other);