Create pkg/front_end/src/base and begin populating with general use classes.

pkg/analyzer/lib/error/error.dart

 // Copyright (c) 2014, 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 analyzer.error.error;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/error/listener.dart';
 import 'package:analyzer/src/dart/scanner/scanner.dart' show ScannerErrorCode;
 import 'package:analyzer/src/error/codes.dart';
 import 'package:analyzer/src/generated/java_core.dart';
 import 'package:analyzer/src/generated/parser.dart' show ParserErrorCode;
 import 'package:analyzer/src/generated/source.dart';
+import 'package:front_end/src/base/errors.dart';
 import 'package:front_end/src/scanner/errors.dart';
 
-export 'package:front_end/src/scanner/errors.dart'
+export 'package:front_end/src/base/errors.dart'
     show ErrorCode, ErrorSeverity, ErrorType;
 
 const List<ErrorCode> errorCodeValues = const [
   //
   // Manually generated. You can mostly reproduce this list by running the
   // following command from the root of the analyzer package:
   //
   // > cat
   //       lib/src/analysis_options/error/option_codes.dart';
   //       lib/src/dart/error/hint_codes.dart';
@@ skipping 869 matching lines @@
 
   @override
   int get hashCode => ordinal;
 
   @override
   int compareTo(ErrorProperty other) => ordinal - other.ordinal;
 
   @override
   String toString() => name;
 }