Make linked-list non-circular.

sdk/lib/collection/linked_list.dart

Index: sdk/lib/collection/linked_list.dart
diff --git a/sdk/lib/collection/linked_list.dart b/sdk/lib/collection/linked_list.dart
index 481f04dde02fd599b8d9ac36d131cb1e360c221b..f1b55bb877482f637e433d3fdf597c5ec8757f40 100644
--- a/sdk/lib/collection/linked_list.dart
+++ b/sdk/lib/collection/linked_list.dart
@@ -258,13 +258,13 @@ abstract class LinkedListEntry<E extends LinkedListEntry<E>> {
   }
 
   /**
-   * Return the succeessor of this element in its linked list.
+   * Return the successor of this element in its linked list.

[Lasse Reichstein Nielsen, 2016/05/26 15:24:10]

Poor "e" doesn't get to be duplicated like all the fancy letters around it :(

    *
    * Returns `null` if there is no successor in the linked list, or if this
    * entry is not currently in any list.
    */
   E get next {
-    if (identical(this, _next)) return null;
+    if (_list == null || identical(_list.first, _next)) return null;
     return _next;
   }
 
@@ -275,7 +275,7 @@ abstract class LinkedListEntry<E extends LinkedListEntry<E>> {
    * entry is not currently in any list.
    */
   E get previous {
-    if (identical(this, _previous)) return null;
+    if (_list == null || identical(this, _list.first)) return null;
     return _previous;
   }