Add MemberEntity.isAbstract and use MemberEntity in function_set.dart

pkg/compiler/lib/src/universe/resolution_world_builder.dart

Index: pkg/compiler/lib/src/universe/resolution_world_builder.dart
diff --git a/pkg/compiler/lib/src/universe/resolution_world_builder.dart b/pkg/compiler/lib/src/universe/resolution_world_builder.dart
index ff1057bc522af1974fabe17ec02f9f2621c907d6..6eaf10427ee0d2689ec3887b4fea41e1654bca7e 100644
--- a/pkg/compiler/lib/src/universe/resolution_world_builder.dart
+++ b/pkg/compiler/lib/src/universe/resolution_world_builder.dart
@@ -764,6 +764,12 @@ abstract class ResolutionWorldBuilderBase
     users.add(mixinApplication);
   }
 
+  void registerUsedElement(MemberEntity element) {
+    if (element.isInstanceMember && !element.isAbstract) {
+      _allFunctions.add(element);
+    }
+  }
+
   ClosedWorld get closedWorldCache {
     assert(isClosed);
     return _closedWorldCache;