Fix a warning.

lib/compiler/implementation/tree/unparser.dart

 // Copyright (c) 2012, 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.
 
 // Returns null if no need to rename a node.
 typedef String Renamer(Node node);
 
 class Unparser implements Visitor {
   Renamer rename;
   StringBuffer sb;
@@ skipping 112 matching lines @@
     // arguments.
     if (node.name is Send) {
       Send send = node.name;
       assert(send is !SendSet);
       if (!send.isOperator) {
         // Looks like a factory method.
         visit(send.receiver);
         sb.add('.');
       } else {
         visit(send.receiver);
-        if (send.selector.token.kind === KEYWORD_TOKEN) sb.add(' ');
+        if (send.selector.asIdentifier().token.kind === KEYWORD_TOKEN) {
+          sb.add(' ');
+        }
       }
       visit(send.selector);
     } else {
       visit(node.name);
     }
     visit(node.parameters);
     visit(node.initializers);
     visit(node.body);
   }
 
@@ skipping 342 matching lines @@
       sb.add(' ');
     }
     visit(node.name);
     if (node.typeParameters !== null) {
       visit(node.typeParameters);
     }
     visit(node.formals);
     add(node.endToken.value);
   }
 }