Improve error-recovery from unmatched braces.

dart/sdk/lib/_internal/compiler/implementation/scanner/token.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.
 
 part of scanner;
 
 const int EOF_TOKEN = 0;
 
 const int KEYWORD_TOKEN = $k;
 const int IDENTIFIER_TOKEN = $a;
@@ skipping 246 matching lines @@
   UnterminatedToken(this.start, int charOffset, this.endOffset)
       : super(charOffset);
 
   String toString() => "UnterminatedToken($start)";
 
   String get assertionMessage => "'$start' isn't terminated.";
 
   int get charCount => endOffset - charOffset;
 }
 
+class UnmatchedToken extends ErrorToken {
+  final BeginGroupToken begin;
+
+  UnmatchedToken(BeginGroupToken begin)
+      : this.begin = begin,
+        super(begin.charOffset);
+
+  String toString() => "UnmatchedToken(${begin.value})";
+
+  String get assertionMessage => "'$begin' isn't closed.";
+}
+
 /**
  * A String-valued token. Represents identifiers, string literals,
  * number literals, comments, and error tokens, using the corresponding
  * precedence info.
  */
 class StringToken extends Token {
   /**
    * The length threshold above which substring tokens are computed lazily.
    *
    * For string tokens that are substrings of the program source, the actual
@@ skipping 424 matching lines @@
 
 const PrecedenceInfo STRING_INTERPOLATION_IDENTIFIER_INFO =
   const PrecedenceInfo('\$', 0,
                        STRING_INTERPOLATION_IDENTIFIER_TOKEN);
 
 const PrecedenceInfo HEXADECIMAL_INFO =
   const PrecedenceInfo('hexadecimal', 0, HEXADECIMAL_TOKEN);
 
 const PrecedenceInfo COMMENT_INFO =
   const PrecedenceInfo('comment', 0, COMMENT_TOKEN);