Add MemberEntity.isAbstract and use MemberEntity in function_set.dart

pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart

Index: pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
diff --git a/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart b/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
index 5c75b9a7633f6e8650acd8e310dfcbb64c5f37b7..78930d74ba5282b1da30f3180c7eef751a8603f8 100644
--- a/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
+++ b/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
@@ -8,6 +8,7 @@ import 'dart:collection' show Queue;
 
 import '../compiler.dart' show Compiler;
 import '../elements/elements.dart';
+import '../elements/entities.dart';
 import '../tree/tree.dart' as ast show Node;
 import '../types/masks.dart'
     show CommonMasks, ContainerTypeMask, MapTypeMask, TypeMask;
@@ -114,9 +115,9 @@ class TypeGraphInferrer implements TypesInferrer {
     }
 
     TypeMask result = const TypeMask.nonNullEmpty();
-    Iterable<Element> elements =
+    Iterable<MemberEntity> elements =
         inferrer.closedWorld.allFunctions.filter(selector, mask);
-    for (Element element in elements) {
+    for (MemberElement element in elements) {
       TypeMask type =
           inferrer.typeOfElementWithSelector(element, selector).type;
       result = result.union(type, inferrer.closedWorld);