Remove a few warnings.

lib/compiler/implementation/scanner/partial_parser.dart

 // Copyright (c) 2011, 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.
 
 class PartialParser extends Parser {
   PartialParser(Listener listener) : super(listener);
 
   Token parseClassBody(Token token) => skipClassBody(token);
 
   Token fullParseClassBody(Token token) => super.parseClassBody(token);
 
   Token parseExpression(Token token) => skipExpression(token);
 
   Token parseArgumentsOpt(Token token) {
     // This method is overridden for two reasons:
     // 1. Avoid generating events for arguments.
     // 2. Avoid calling skip expression for each argument (which doesn't work).
     if (optional('(', token)) {
-      return token.endGroup.next;
+      BeginGroupToken begin = token;
+      return begin.endGroup.next;
     } else {
       return token;
     }
   }
 
   Token skipExpression(Token token) {
     while (true) {
       final kind = token.kind;
       final value = token.stringValue;
       if ((kind === EOF_TOKEN) ||
@@ skipping 81 matching lines @@
         return token;
       }
       return listener.unexpected(token);
     }
     BeginGroupToken beginGroupToken = token;
     Token endToken = beginGroupToken.endGroup;
     listener.endFormalParameters(0, token, endToken);
     return endToken.next;
   }
 }