Make linked-list non-circular.

This bug was introduced when making the collection library strong-mode clean.

Fixes #26522
BUG= http://dartbug.com/26522
R=lrn@google.com

Review URL: https://codereview.chromium.org/2015513006 .

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.
    *
    * 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;
   }
 

tests/lib/collection/linked_list_test.dart

@@ -14,6 +14,55 @@ class MyEntry extends LinkedListEntry<MyEntry> {
 }
 
 
+testPreviousNext() {
+  var list = new LinkedList<MyEntry>();
+  Expect.throws(() => list.first);
+  Expect.throws(() => list.last);
+  Expect.equals(0, list.length);
+
+  for (int i = 0; i < 3; i++) {
+    list.add(new MyEntry(i));
+  }
+  Expect.equals(3, list.length);
+
+  var entry = list.first;
+  Expect.isNull(entry.previous);
+  Expect.equals(0, entry.value);
+  entry = entry.next;
+  Expect.equals(1, entry.value);
+  entry = entry.next;
+  Expect.equals(2, entry.value);
+  Expect.isNull(entry.next);
+  entry = entry.previous;
+  Expect.equals(1, entry.value);
+  entry = entry.previous;
+  Expect.equals(0, entry.value);
+  Expect.isNull(entry.previous);
+}
+
+testUnlinked() {
+  var unlinked = new MyEntry(0);
+  Expect.isNull(unlinked.previous);
+  Expect.isNull(unlinked.next);
+  var list = new LinkedList<MyEntry>();
+  list.add(unlinked);
+  Expect.isNull(unlinked.previous);
+  Expect.isNull(unlinked.next);
+  list.remove(unlinked);
+  Expect.isNull(unlinked.previous);
+  Expect.isNull(unlinked.next);
+  list.add(unlinked);
+  list.add(new MyEntry(1));
+  Expect.isNull(unlinked.previous);
+  Expect.equals(1, unlinked.next.value);
+  list.remove(unlinked);
+  Expect.isNull(unlinked.previous);
+  Expect.isNull(unlinked.next);
+  list.add(unlinked);
+  Expect.isNull(unlinked.next);
+  Expect.equals(1, unlinked.previous.value);
+}
+
 testInsert() {
   // Insert last.
   var list = new LinkedList<MyEntry>();
@@ -168,6 +217,8 @@ testConcurrentModificationError() {
 }
 
 main() {
+  testPreviousNext();
+  testUnlinked();
   testInsert();
   testRemove();
   testBadAdd();