Better documentation on Sets and Maps, and more.

sdk/lib/collection/hash_map.dart

Index: sdk/lib/collection/hash_map.dart
diff --git a/sdk/lib/collection/hash_map.dart b/sdk/lib/collection/hash_map.dart
index 431c89f5dc21dcc2eecb501073fe495d326a0c1a..2aea9c963205e3a520336826abcda3596d94151e 100644
--- a/sdk/lib/collection/hash_map.dart
+++ b/sdk/lib/collection/hash_map.dart
@@ -4,6 +4,17 @@
 
 part of dart.collection;
 
+/**
+ * A hash-table based implementation of [Map].
+ *
+ * The keys of a `HashMap` must have consistent [Object.operator==]
+ * and [Object.hashCode] implementations. This means that the `==` operator
+ * must define a stable equivalence relation on the keys (reflexive,
+ * anti-symmetric, transitive, and consistent over time), and that `hashCode`
+ * must be the same for objects that are considered equal by `==`.
+ *
+ * The map allows `null` as a key.
+ */
 class HashMap<K, V> implements Map<K, V> {
   external HashMap();