Add MapMixin to dart:collection.

sdk/lib/collection/maps.dart

Index: sdk/lib/collection/maps.dart
diff --git a/sdk/lib/collection/maps.dart b/sdk/lib/collection/maps.dart
index ef937b35548681406110bc609b19e0c7514e5a0f..1e4214fd2cbbc3e0fada8f32b7e2ca0ec8f981cc 100644
--- a/sdk/lib/collection/maps.dart
+++ b/sdk/lib/collection/maps.dart
@@ -20,9 +20,26 @@ part of dart.collection;
  * A more efficient implementation is usually possible by overriding
  * some of the other members as well.
  */
-abstract class MapBase<K, V> implements Map<K, V> {
-  MapBase();  // Prevents use as mixin.
+abstract class MapBase<K, V> = Object with MapMixin<K, V>;
 
+
+/**
+ * Mixin implementing a [Map].
+ *
+ * This mixin has a basic implementation of all but five of the members of
+ * [Map].
+ * A basic `Map` class can be implemented by mixin in this class and

[kevmoo, 2014/04/29 13:41:11]

include a blank line after the first sentence.

+ * implementing `keys`, `operator[]`, `operator[]=`, `remove` and `clear`.
+ * The remaining operations are implemented in terms of these five.
+ *
+ * The `keys` iterable should have efficient [length] and [contains]
+ * operations, and it should catch concurrent modifications of the keys
+ * while iterating.
+ *
+ * A more efficient implementation is usually possible by overriding
+ * some of the other members as well.
+ */
+abstract class MapMixin<K, V> implements Map<K, V> {
   Iterable<K> get keys;
   V operator[](Object key);
   operator []=(K key, V value);