Do not rely on the PackageGraph.objectClass late field

Author: srawlins

lib/src/model/mixin.dart

@@ -16,10 +16,8 @@ class Mixin extends InheritingContainer {
   final MixinElement element;
 
   late final List<ParameterizedElementType> superclassConstraints = [
-    ...element.superclassConstraints
-        .map((InterfaceType i) =>
-            getTypeFor(i, library) as ParameterizedElementType)
-        .where((t) => t.modelElement != packageGraph.objectClass)
+    ...element.superclassConstraints.where((e) => !e.isDartCoreObject).map(
+        (InterfaceType i) => getTypeFor(i, library) as ParameterizedElementType)
   ];
 
   @override

lib/src/model/package_graph.dart

@@ -204,6 +204,7 @@ class PackageGraph with CommentReferable, Nameable {
   final Map<Element, ModelNode> _modelNodes = {};
 
   /// The Object class declared in the Dart SDK's 'dart:core' library.
+  // TODO(srawlins): I think nothing depends on this any longer; remove.
   late InheritingContainer objectClass;
 
   /// Populate's [_modelNodes] with elements in [resolvedLibrary].