Add MemberEntity.isAbstract and use MemberEntity in function_set.dart

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

Index: pkg/compiler/lib/src/elements/entities.dart
diff --git a/pkg/compiler/lib/src/elements/entities.dart b/pkg/compiler/lib/src/elements/entities.dart
index 147b797e36b35a7a33350a7a65c5ac02a6e64540..33d5561c5d9dce8627222d3fd7916a428f7c75e1 100644
--- a/pkg/compiler/lib/src/elements/entities.dart
+++ b/pkg/compiler/lib/src/elements/entities.dart
@@ -92,6 +92,10 @@ abstract class MemberEntity extends Entity {
   /// 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;