Add MemberEntity.isAbstract and use MemberEntity in function_set.dart

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

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of world_builder;
 
 abstract class ResolutionWorldBuilder implements WorldBuilder, OpenWorld {
   /// Set of all local functions in the program. Used by the mirror tracking
   /// system to find all live closure instances.
   Iterable<Local> get localFunctions;
@@ skipping 746 matching lines @@
   }
 
   void registerMixinUse(ClassEntity mixinApplication, ClassEntity mixin) {
     // TODO(johnniwinther): Add map restricted to live classes.
     // We don't support patch classes as mixin.
     Set<ClassEntity> users =
         _mixinUses.putIfAbsent(mixin, () => new Set<ClassEntity>());
     users.add(mixinApplication);
   }
 
+  void registerUsedElement(MemberEntity element) {
+    if (element.isInstanceMember && !element.isAbstract) {
+      _allFunctions.add(element);
+    }
+  }
+
   ClosedWorld get closedWorldCache {
     assert(isClosed);
     return _closedWorldCache;
   }
 
   @override
   bool isMemberUsed(MemberEntity member) {
     if (member.isInstanceMember) {
       _MemberUsage usage = _instanceMemberUsage[member];
       if (usage != null && usage.hasUse) return true;
     }
     _StaticMemberUsage usage = _staticMemberUsage[member];
     return usage != null && usage.hasUse;
   }
 }