Add MemberEntity.isAbstract and use MemberEntity in function_set.dart

pkg/compiler/lib/src/dump_info.dart

Index: pkg/compiler/lib/src/dump_info.dart
diff --git a/pkg/compiler/lib/src/dump_info.dart b/pkg/compiler/lib/src/dump_info.dart
index feae6ee36bba8fa633057d319ffd54f37a7b82b0..3194e57e7b2cd434d83047918a7a3b4af9bfc80c 100644
--- a/pkg/compiler/lib/src/dump_info.dart
+++ b/pkg/compiler/lib/src/dump_info.dart
@@ -463,7 +463,7 @@ class DumpInfoTask extends CompilerTask implements InfoReporter {
         new WorldImpactVisitorImpl(visitDynamicUse: (dynamicUse) {
           selections.addAll(closedWorld.allFunctions
               .filter(dynamicUse.selector, dynamicUse.mask)
-              .map((e) => new Selection(e, dynamicUse.mask)));
+              .map((MemberElement e) => new Selection(e, dynamicUse.mask)));
         }, visitStaticUse: (staticUse) {
           selections.add(new Selection(staticUse.element, null));
         }),