Add MemberEntity.isAbstract and use MemberEntity in function_set.dart

pkg/compiler/lib/src/elements/entities.dart

 // Copyright (c) 2016, 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 entities;
 
 import '../common.dart';
 
 /// Abstract interface for entities.
 ///
@@ skipping 74 matching lines @@
 
   /// Whether this is a setter.
   bool get isSetter;
 
   /// Whether this member is assignable, i.e. a non-final field.
   bool get isAssignable;
 
   /// Whether this member is constant, i.e. a constant field or constructor.
   bool get isConst;
 
+  /// Whether this member is abstract, i.e. an abstract method, getter or
+  /// setter.
+  bool get isAbstract;
+
   /// The enclosing class if this is a constructor, instance member or
   /// static member of a class.
   ClassEntity get enclosingClass;
 
   /// The enclosing library if this is a library member, otherwise the
   /// enclosing library of the [enclosingClass].
   LibraryEntity get library;
 }
 
 /// Stripped down super interface for field like entities.
@@ skipping 42 matching lines @@
   Entity get executableContext;
 
   /// The outermost member that contains this element.
   ///
   /// For top level, static or instance members, the member context is the
   /// element itself. For parameters, local variables and nested closures, the
   /// member context is the top level, static or instance member in which it is
   /// defined.
   MemberEntity get memberContext;
 }