Replace ClassHierarchy.classes with getOrderedClasses().

pkg/kernel/lib/class_hierarchy.dart

Index: pkg/kernel/lib/class_hierarchy.dart
diff --git a/pkg/kernel/lib/class_hierarchy.dart b/pkg/kernel/lib/class_hierarchy.dart
index 34bf1277580f7850ec2fec6bacaf9d30a210d904..7fddd6394f152a6e5fa71629c876040e33e677bc 100644
--- a/pkg/kernel/lib/class_hierarchy.dart
+++ b/pkg/kernel/lib/class_hierarchy.dart
@@ -16,12 +16,14 @@ abstract class ClassHierarchy {
   factory ClassHierarchy(Program program) =>
       new ClosedWorldClassHierarchy(program);
 
-  /// All classes in the program.
+  /// Given the [unordered] classes, return them in such order that classes
+  /// occur after their super classes.
   ///
-  /// The iterable is ordered so that classes occur after their super classes.
-  Iterable<Class> get classes;
+  /// If [unordered] does not include all superclasses, they might still be
+  /// returned, it is up to the client to filter them out if required.
+  Iterable<Class> getOrderedClasses(Iterable<Class> unordered);
 
-  /// Returns the index of [class_] in the [classes] list.
+  /// Returns the unique index of the [class_].
   int getClassIndex(Class class_);
 
   /// True if the program contains another class that is a subtype of given one.
@@ -137,6 +139,9 @@ class ClosedWorldClassHierarchy implements ClassHierarchy {
   @override
   int getClassIndex(Class class_) => _infoFor[class_].topologicalIndex;
 
+  @override
+  Iterable<Class> getOrderedClasses(Iterable<Class> unordered) => classes;
+
   /// True if [subclass] inherits from [superclass] though zero or more
   /// `extends` relationships.
   bool isSubclassOf(Class subclass, Class superclass) {