cleanup fasta token classes

pkg/front_end/lib/src/fasta/scanner/error_token.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 licenset hat can be found in the LICENSE file.
 
 library dart_scanner.error_token;
 
-import '../../scanner/token.dart' show TokenType, TokenWithComment;
+import '../../scanner/token.dart' show BeginToken, TokenType, TokenWithComment;
 
 import '../fasta_codes.dart'
     show
         FastaCode,
         codeAsciiControlCharacter,
         codeEncoding,
         codeExpectedHexDigit,
         codeMissingExponent,
         codeNonAsciiIdentifier,
         codeNonAsciiWhitespace,
         codeUnexpectedDollarInString,
         codeUnmatchedToken,
         codeUnterminatedComment,
         codeUnterminatedString,
         codeUnterminatedToken;
 
-import '../scanner.dart'
-    show BeginGroupToken, Token, unicodeReplacementCharacter;
+import '../scanner.dart' show Token, unicodeReplacementCharacter;
 
 ErrorToken buildUnexpectedCharacterToken(int character, int charOffset) {
   if (character < 0x1f) {
     return new AsciiControlCharacterToken(character, charOffset);
   }
   switch (character) {
     case unicodeReplacementCharacter:
       return new EncodingErrorToken(charOffset);
 
     /// See [General Punctuation]
@@ skipping 43 matching lines @@
   String get assertionMessage;
 
   FastaCode get errorCode;
 
   int get character => null;
 
   String get start => null;
 
   int get endOffset => null;
 
-  BeginGroupToken get begin => null;
+  BeginToken get begin => null;
 
   @override
   Token copy() {
     throw 'unsupported operation';
   }
 }
 
 /// Represents an encoding error.
 class EncodingErrorToken extends ErrorToken {
   EncodingErrorToken(int charOffset) : super(charOffset);
@@ skipping 104 matching lines @@
         return codeUnterminatedToken;
     }
   }
 }
 
 /// Represents an open brace without a matching close brace.
 ///
 /// In this case, brace means any of `(`, `{`, `[`, and `<`, parenthesis, curly
 /// brace, square brace, and angle brace, respectively.
 class UnmatchedToken extends ErrorToken {
-  final BeginGroupToken begin;
+  final BeginToken begin;
 
-  UnmatchedToken(BeginGroupToken begin)
+  UnmatchedToken(BeginToken begin)
       : this.begin = begin,
         super(begin.charOffset);
 
   String toString() => "UnmatchedToken(${begin.lexeme})";
 
   String get assertionMessage => "'$begin' isn't closed.";
 
   FastaCode get errorCode => codeUnmatchedToken;
 }