Add MemberEntity.isAbstract and use MemberEntity in function_set.dart

pkg/compiler/lib/src/inferrer/closure_tracer.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 compiler.src.inferrer.closure_tracer;
 
 import '../common/names.dart' show Names;
 import '../elements/elements.dart';
 import '../js_backend/backend_helpers.dart';
 import '../types/types.dart' show TypeMask;
@@ skipping 72 matching lines @@
       }
     }
     if (called.isGetter &&
         info.selector != null &&
         info.selector.isCall &&
         inferrer.types.getInferredTypeOf(called) == currentUser) {
       // This node can be a closure call as well. For example, `foo()`
       // where `foo` is a getter.
       _registerCallForLaterAnalysis(info);
     }
-    if (_checkIfFunctionApply(called) &&
+    if (called is MemberElement &&
+        _checkIfFunctionApply(called) &&
         info.arguments != null &&
         info.arguments.contains(currentUser)) {
       _tagAsFunctionApplyTarget("static call");
     }
   }
 
   bool _checkIfCurrentUser(element) =>
       inferrer.types.getInferredTypeOf(element) == currentUser;
 
-  bool _checkIfFunctionApply(Element element) {
-    return element is MemberElement &&
-        compiler.commonElements.isFunctionApplyMethod(element);
+  bool _checkIfFunctionApply(MemberElement element) {
+    return compiler.commonElements.isFunctionApplyMethod(element);
   }
 
   @override
   visitDynamicCallSiteTypeInformation(DynamicCallSiteTypeInformation info) {
     super.visitDynamicCallSiteTypeInformation(info);
     if (info.selector.isCall) {
       if (info.arguments.contains(currentUser)) {
         if (!info.targets.every((element) => element.isFunction)) {
           bailout('Passed to a closure');
         }
@@ skipping 18 matching lines @@
   @override
   visitStaticCallSiteTypeInformation(StaticCallSiteTypeInformation info) {
     super.visitStaticCallSiteTypeInformation(info);
     if (info.calledElement == tracedElements.first &&
         info.selector != null &&
         info.selector.isGetter) {
       addNewEscapeInformation(info);
     }
   }
 }