Add MemberEntity.isAbstract and use MemberEntity in function_set.dart

pkg/compiler/lib/src/universe/element_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;
 
 /// [ResolutionEnqueuerWorldBuilder] based on the [Element] model.
 class ElementResolutionWorldBuilder extends ResolutionWorldBuilderBase {
   /// Used for testing the new more precise computation of instantiated types
   /// and classes.
@@ skipping 219 matching lines @@
         commonElements: _commonElements,
         resolutionWorldBuilder: this,
         functionSetBuilder: _allFunctions,
         allTypedefs: _allTypedefs,
         mixinUses: _mixinUses,
         typesImplementedBySubclasses: typesImplementedBySubclasses,
         classHierarchyNodes: _classHierarchyNodes,
         classSets: _classSets);
   }
 
-  void registerUsedElement(MemberElement element) {
-    if (element.isInstanceMember && !element.isAbstract) {
-      _allFunctions.add(element);
-    }
-  }
-
   @override
   void registerMixinUse(
       MixinApplicationElement mixinApplication, ClassElement mixin) {
     assert(mixin.isDeclaration);
     super.registerMixinUse(mixinApplication, mixin);
   }
 }