Make Iterable more powerful (and lazy).

sdk/lib/core/map.dart

Index: sdk/lib/core/map.dart
diff --git a/sdk/lib/core/map.dart b/sdk/lib/core/map.dart
index 60db6ce71c70ec078f0fd4dcdcd2673803d0c4e4..4c3787903184b0148bf701e8eb0523e82886d092 100644
--- a/sdk/lib/core/map.dart
+++ b/sdk/lib/core/map.dart
@@ -67,14 +67,20 @@ abstract class Map<K, V> {
   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.
@@ -85,6 +91,13 @@ abstract class Map<K, V> {
    * 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;
 }
 
 /**