Make Iterable more powerful (and lazy).

sdk/lib/core/map.dart

 // Copyright (c) 2011, 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.
 
 /**
  * A [Map] is an associative container, mapping a key to a value.
  * Null values are supported, but null keys are not.
  */
 abstract class Map<K, V> {
   /**
@@ skipping 49 matching lines @@
    * Removes all pairs from the map.
    */
   void clear();
 
   /**
    * Applies [f] to each {key, value} pair of the map.
    */
   void forEach(void f(K key, V value));
 
   /**
-   * Returns a collection containing all the keys in the map.
+   * Returns a [Queryable] iterating over all the keys in the map.
+   *
+   * The iteration-order is guaranteed to be the same for [keys] and [values].
+   * That is, [values] is equivalent to [:keys.mappedBy((key) => this[key]):];
    */
-  Collection<K> get keys;
+  Queryable<K> get keys;
 
   /**
-   * Returns a collection containing all the values in the map.
+   * Returns a [Queryable] iterating over all the value of the map.
+   *
+   * The iteration-order is guaranteed to be the same for [keys] and [values].
+   * That is, [values] is equivalent to [:keys.mappedBy((key) => this[key]):];
    */
-  Collection<V> get values;
+  Queryable<V> get values;
 
   /**
    * The number of {key, value} pairs in the map.
    */
   int get length;
 
   /**
    * Returns true if there is no {key, value} pair in the map.
    */
   bool get isEmpty;
+
+  /**
+   * Returns a read-only view of [this].
+   *
+   * The returned [Map] is backed by [this] but cannot change [this].
+   */
+  Map<K, V> get view;
 }
 
 /**
  * Hash map version of the [Map] interface. A [HashMap] does not
  * provide any guarantees on the order of keys and values in [keys]
  * and [values].
  */
 abstract class HashMap<K, V> extends Map<K, V> {
   /**
    * Creates a map with the default implementation.
@@ skipping 419 matching lines @@
 
   void clear() {
     _map.clear();
     _list.clear();
   }
 
   String toString() {
     return Maps.mapToString(this);
   }
 }