Make Mirror.simpleName unique.

pkg/dartdoc/lib/src/mirrors/util.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('util');
 
 /**
  * An abstract map implementation. This class can be used as a superclass for
  * implementing maps, requiring only the further implementation of the
  * [:operator []:], [:forEach:] and [:length:] methods to provide a fully
@@ skipping 127 matching lines @@
   int get length {
     var count = 0;
     forEach((k,v) {
       count++;
     });
     return count;
   }
 
   Vout operator [](K key) {
     if (key is K) {
-      return _filter(_map[key]);
+      Vin value = _map[key];
+      if (value !== null) {
+        return _filter(value);
+      }
     }
     return null;
   }
 
   void forEach(void f(K key, Vout value)) {
     _map.forEach((K k, Vin v) {
       var value = _filter(v);
       if (value !== null) {
         f(k, value);
       }
     });
   }
 }