Fix analyzer warning in analyzer2dart.

pkg/analyzer2dart/lib/src/modely.dart

 // Copyright (c) 2014, 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.
 
 part of analyzer2dart.element_converter;
 
 
 /// Base [dart2js.Element] implementation for converted analyzer elements.
 class ElementY extends dart2js.Element {
   final ElementConverter converter;
@@ skipping 343 matching lines @@
   computeSignature(_) => unsupported('computeSignature');
 
   @override
   get memberContext => unsupported('memberContext');
 
   @override
   get functionSignature => unsupported('functionSignature');
 
   @override
   bool get hasFunctionSignature => unsupported('hasFunctionSignature');
+
+  @override
+  get asyncMarker => unsupported('asyncMarker');
 }
 
 class TopLevelFunctionElementY extends ElementY
     with AnalyzableElementY,
          AstElementY,
          TopLevelElementMixin,
          FunctionElementMixin
     implements dart2js.FunctionElement {
   analyzer.FunctionElement get element => super.element;
 
@@ skipping 360 matching lines @@
 
   @override
   bool get isRedirectingFactory => unsupported('isRedirectingFactory');
 
   @override
   get nestedClosures => unsupported('nestedClosures');
 
   @override
   get type => unsupported('type');
 }