Move parser helper classes to own files and clean them up.

pkg/front_end/lib/src/fasta/parser/parser_error.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.parser_error;
+
+import '../fasta_codes.dart' show Message;
+
+import '../scanner.dart' show Token;
+
+class ParserError {
+  /// Character offset from the beginning of file where this error starts.
+  final int beginOffset;
+
+  /// Character offset from the beginning of file where this error ends.
+  final int endOffset;
+
+  final Message message;
+
+  ParserError(this.beginOffset, this.endOffset, this.message);
+
+  ParserError.fromTokens(Token begin, Token end, Message message)
+      : this(begin.charOffset, end.charOffset + end.charCount, message);
+
+  String toString() => "@${beginOffset}: ${message.message}\n${message.tip}";
+}