Revert "Remove unmodifiable map wrappers that could instead use UnmodifiableMapView."

sdk/lib/_internal/compiler/implementation/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 dart2js.mirrors.util;
 
 import 'dart:collection' show Maps;
 
 /**
  * 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
  * implemented immutable map.
  */
-// TODO(lrn): Consider using UnmodifiableBaseMap/UnmodifiableMapWrapper
-// for these classes, or just rewrite for a bit more efficiency.
 abstract class AbstractMap<K, V> implements Map<K, V> {
   AbstractMap();
 
   AbstractMap.from(Map<K, V> other) {
     other.forEach((k,v) => this[k] = v);
   }
 
   void operator []=(K key, value) {
     throw new UnsupportedError('[]= is not supported');
   }
@@ skipping 148 matching lines @@
 
   void forEach(void f(K key, Vout value)) {
     _map.forEach((K k, Vin v) {
       var value = _filter(v);
       if (value != null) {
         f(k, value);
       }
     });
   }
 }