Add closeBraceTokenFor and use it instead of BeginToken.

pkg/front_end/lib/src/fasta/parser/util.dart

 // Copyright (c) 2017, 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 fasta.parser.util;
 
 import '../scanner.dart' show Token;
 
+import '../../scanner/token.dart' show BeginToken;
+
 /// Returns true if [token] is the symbol or keyword [value].
 bool optional(String value, Token token) {
   return identical(value, token.stringValue);
 }
+
+/// Returns the close brace, bracket, or parenthesis of [left]. For '<', it may
+/// return null.
+Token closeBraceTokenFor(BeginToken left) => left.endToken;