Add ClassSet to support ClassWorld.strictSubtypesOf

pkg/compiler/lib/src/serialization/modelz.dart

Index: pkg/compiler/lib/src/serialization/modelz.dart
diff --git a/pkg/compiler/lib/src/serialization/modelz.dart b/pkg/compiler/lib/src/serialization/modelz.dart
index a0ba0a8d5db71215630c33e98619e37e37229745..8eb87d4e23e2727b0666c02b3c35ede6141cca2e 100644
--- a/pkg/compiler/lib/src/serialization/modelz.dart
+++ b/pkg/compiler/lib/src/serialization/modelz.dart
@@ -940,6 +940,11 @@ class ClassElementZ extends DeserializedElementZ
 
   @override
   ClassElement get superclass => supertype != null ? supertype.element : null;
+
+  @override
+  void ensureResolved(Compiler compiler) {
+    compiler.world.registerClass(this);
+  }
 }
 
 abstract class ConstructorElementZ extends DeserializedElementZ
@@ -1280,9 +1285,6 @@ abstract class TypeDeclarationMixin<T extends GenericType>
 
   @override
   bool get isResolved => true;
-
-  @override
-  void ensureResolved(Compiler compiler) {}
 }
 
 class TypedefElementZ extends DeserializedElementZ
@@ -1318,6 +1320,9 @@ class TypedefElementZ extends DeserializedElementZ
   }
 
   @override
+  void ensureResolved(Compiler compiler) {}
+
+  @override
   void checkCyclicReference(Compiler compiler) {}
 }