Move errors for bad imports and exports to the directive

pkg/analyzer/lib/src/generated/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.src.generated.error;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart' show AstNode;
 import 'package:analyzer/dart/ast/token.dart';
@@ skipping 67 matching lines @@
 
   /**
    * The correction to be displayed for this error, or `null` if there is no
    * correction information for this error.
    */
   String _correction;
 
   /**
    * The source in which the error occurred, or `null` if unknown.
    */
-  Source source;
+  final Source source;
 
   /**
    * The character offset from the beginning of the source (zero based) where
    * the error occurred.
    */
   int offset = 0;
 
   /**
    * The number of characters from the offset to the end of the source which
    * encompasses the compilation error.
@@ skipping 5613 matching lines @@
    * Initialize a newly created error code to have the given [name].
    */
   const TodoCode(String name) : super(name, "{0}");
 
   @override
   ErrorSeverity get errorSeverity => ErrorSeverity.INFO;
 
   @override
   ErrorType get type => ErrorType.TODO;
 }