Retain elements a finer granularity than library.

dart/sdk/lib/_internal/compiler/compiler.dart

 // Copyright (c) 2012, 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 compiler;
 
 import 'dart:async';
 import 'implementation/apiimpl.dart';
 
 // Unless explicitly allowed, passing [:null:] for any argument to the
@@ skipping 122 matching lines @@
    * Specification" [http://www.dartlang.org/docs/spec/].
    */
   static const Diagnostic WARNING = const Diagnostic(2, 'warning');
 
   /**
    * Any other warning that is not covered by [WARNING].
    */
   static const Diagnostic HINT = const Diagnostic(4, 'hint');
 
   /**
-   * Informational messages.
+   * Additional information about the preceding non-info diagnostic from the
+   * compiler.
+   *
+   * For example, consider a duplicated definition. The compiler first emits a
+   * message about the duplicated definition, then emits an info message about
+   * the location of the existing definition.
    */
   static const Diagnostic INFO = const Diagnostic(8, 'info');
 
   /**
    * Informational messages that shouldn't be printed unless
    * explicitly requested by the user of a compiler.
    */
   static const Diagnostic VERBOSE_INFO = const Diagnostic(16, 'verbose info');
 
   /**
@@ skipping 13 matching lines @@
   final String name;
 
   /**
    * This constructor is not private to support user-defined
    * diagnostic kinds.
    */
   const Diagnostic(this.ordinal, this.name);
 
   String toString() => name;
 }