Make LinkedHashMap also have a factory constructor and be customizable

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 3aadab231762b62d3599d4cafce6ff22594dc8b0..bcca966bba5504920f033b4be915596118f83024 100644
--- a/sdk/lib/collection/hash_map.dart
+++ b/sdk/lib/collection/hash_map.dart
@@ -41,6 +41,14 @@ abstract class HashMap<K, V> implements Map<K, V> {
    * for keys in order to place them in the hash table. If it is omitted, the
    * key's own [Object.hashCode] is used.
    *
+   * If using methods like [operator[]], [remove] and [containsKey] together
+   * with a custom equality and hashcode, an extra `isValidKey` function
+   * can be supplied. This function is called before calling [equals] or
+   * [hashCode] with an argument that may not be a [K] instance, and if the
+   * call returns false, the key is assumed to not be in the set.
+   * The [isValidKey] function defaults to just testing if the object is a
+   * [K] instance.
+   *
    * The used `equals` and `hashCode` method should always be consistent,
    * so that if `equals(a, b)` then `hashCode(a) == hashCode(b)`. The hash
    * of an object, or what it compares equal to, should not change while the
@@ -50,7 +58,9 @@ abstract class HashMap<K, V> implements Map<K, V> {
    * you also want to supply the other. The only common exception is to pass
    * [identical] as the equality and use the default hash code.
    */
-  external factory HashMap({bool equals(K key1, K key2), int hashCode(K key)});
+  external factory HashMap({bool equals(K key1, K key2),
+                            int hashCode(K key),
+                            bool isValidKey(potentialKey)});
 
   /**
    * Creates a [HashMap] that contains all key value pairs of [other].