Add SyntacticEntity, an interface shared by AstNode and Token.

pkg/analyzer/lib/src/generated/element_resolver.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.
 
 library analyzer.src.generated.element_resolver;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
@@ skipping 2594 matching lines @@
    */
   SyntheticIdentifier(this.name, this.targetIdentifier);
 
   @override
   Token get beginToken => null;
 
   @override
   Element get bestElement => null;
 
   @override
-  Iterable get childEntities {
+  Iterable<SyntacticEntity> get childEntities {
     // Should never be called, since a SyntheticIdentifier never appears in the
     // AST--it is just used for lookup.
     assert(false);
     return new ChildEntities();
   }
 
   @override
   Token get endToken => null;
 
   @override
@@ skipping 10 matching lines @@
 
   @override
   Element get staticElement => null;
 
   @override
   dynamic/*=E*/ accept/*<E>*/(AstVisitor/*<E>*/ visitor) => null;
 
   @override
   void visitChildren(AstVisitor visitor) {}
 }