Use Set<ClassEntity> instead of Set<MixinApplicationElement>

pkg/compiler/lib/src/world.dart

Index: pkg/compiler/lib/src/world.dart
diff --git a/pkg/compiler/lib/src/world.dart b/pkg/compiler/lib/src/world.dart
index 0a0a2f3a491fa6f5d4f866848e9f212768725505..47fe973ad1106d22785484477ce53b616e8df975 100644
--- a/pkg/compiler/lib/src/world.dart
+++ b/pkg/compiler/lib/src/world.dart
@@ -841,7 +841,7 @@ class ClosedWorldImpl implements ClosedWorld, ClosedWorldRefiner {
           if (isInstantiated(mixinApplication)) {
             uses.add(mixinApplication);
           } else if (mixinApplication.isNamedMixinApplication) {
-            Set<MixinApplicationElement> next = _mixinUses[mixinApplication];
+            Set<ClassEntity> next = _mixinUses[mixinApplication];
             if (next != null) {
               next.forEach(addLiveUse);
             }