Use ClassHierarchy.applyChanges() in MixinFullResolution.

pkg/kernel/lib/transformations/mixin_full_resolution.dart

Index: pkg/kernel/lib/transformations/mixin_full_resolution.dart
diff --git a/pkg/kernel/lib/transformations/mixin_full_resolution.dart b/pkg/kernel/lib/transformations/mixin_full_resolution.dart
index 42555e5f335d886c99c546db5aeaac5f2d4b34cb..44b166af26fee507423239fec3797cc1c0a53151 100644
--- a/pkg/kernel/lib/transformations/mixin_full_resolution.dart
+++ b/pkg/kernel/lib/transformations/mixin_full_resolution.dart
@@ -53,11 +53,8 @@ class MixinFullResolution {
       }
     }
 
-    // If there are transformed classes, we need to update hierarchy.
-    if (transformedClasses.isNotEmpty) {
-      var program = libraries.first.enclosingProgram;
-      hierarchy = new ClosedWorldClassHierarchy(program);
-    }
+    // We might need to update the class hierarchy.
+    hierarchy = hierarchy.applyChanges(transformedClasses);
 
     // Resolve all super call expressions and super initializers.
     for (var library in libraries) {