Reapply "New implementation of {,Linked}Hash{Set,Map}."

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 8b27b3e07bc4a95a9567f0aee70be0cec6e73258..de3807f766be81f55171b59bca3291ad29167fb8 100644
--- a/sdk/lib/core/map.dart
+++ b/sdk/lib/core/map.dart
@@ -47,6 +47,9 @@ abstract class Map<K, V> {
    * If [key] is not associated to a value, calls [ifAbsent] and
    * updates the map by mapping [key] to the value returned by
    * [ifAbsent]. Returns the value in the map.
+   *
+   * It is an error to add or remove keys from map during the call to
+   * [ifAbsent].
    */
   V putIfAbsent(K key, V ifAbsent());
 
@@ -65,6 +68,8 @@ abstract class Map<K, V> {
 
   /**
    * Applies [f] to each {key, value} pair of the map.
+   *
+   * It is an error to add or remove keys from the map during iteration.
    */
   void forEach(void f(K key, V value));