Fix NPE in parser.

dart/frog/leg/scanner/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.
 
 /**
  * An event generating parser of Dart programs. This parser expects
  * all tokens in a linked list.
  */
 class Parser {
   final Listener listener;
@@ skipping 299 matching lines @@
     return parseStuff(token,
                       (t) => listener.beginTypeArguments(t),
                       (t) => parseType(t),
                       (c, bt, et) => listener.endTypeArguments(c, bt, et),
                       (t) => listener.handleNoTypeArguments(t));
   }
 
   Token parseTypeVariablesOpt(Token token) {
     if (optional('<', token)) {
       BeginGroupToken beginGroupToken = token;
-      token = beginGroupToken.endGroup.next;
+      // TODO(ahe): Parse type variables.
+
+      // For now, skip to the matching '>' if it exists. Otherwise,
+      // don't advance and assume the caller will report some kind of
+      // error.
+      if (beginGroupToken.endGroup !== null) {
+        token = beginGroupToken.endGroup.next;
+      }
     }
     listener.handleNoTypeVariables(token);
     return token;
   }
 
   Token parseTypeVariablesOptX(Token token) {
     return parseStuff(token,
                       (t) => listener.beginTypeVariables(t),
                       (t) => parseTypeVariable(t),
                       (c, bt, et) => listener.endTypeVariables(c, bt, et),
@@ skipping 1282 matching lines @@
     }
     listener.handleContinueStatement(hasTarget, continueKeyword, token);
     return expectSemicolon(token);
   }
 
   Token parseEmptyStatement(Token token) {
     listener.handleEmptyStatement(token);
     return expectSemicolon(token);
   }
 }