Add MemberEntity.isAbstract and use MemberEntity in function_set.dart

pkg/compiler/lib/src/dump_info.dart

 // Copyright (c) 2013, 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.
 
 library dump_info;
 
 import 'dart:convert'
     show ChunkedConversionSink, JsonEncoder, StringConversionSink;
 
 import 'package:dart2js_info/info.dart';
@@ skipping 445 matching lines @@
     WorldImpact impact = impacts[element];
     if (impact == null) return const <Selection>[];
 
     var selections = <Selection>[];
     compiler.impactStrategy.visitImpact(
         element,
         impact,
         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));
         }),
         IMPACT_USE);
     return selections;
   }
 
   // Returns true if we care about tracking the size of
   // this node.
   bool isTracking(jsAst.Node code) {
@@ skipping 140 matching lines @@
 
     ChunkedConversionSink<Object> sink = encoder.startChunkedConversion(
         new StringConversionSink.fromStringSink(buffer));
     sink.add(new AllInfoJsonCodec().encode(result));
     compiler.reporter.reportInfo(NO_LOCATION_SPANNABLE, MessageKind.GENERIC, {
       'text': "View the dumped .info.json file at "
           "https://dart-lang.github.io/dump-info-visualizer"
     });
   }
 }