Make linked-list non-circular.

tests/lib/collection/linked_list_test.dart

Index: tests/lib/collection/linked_list_test.dart
diff --git a/tests/lib/collection/linked_list_test.dart b/tests/lib/collection/linked_list_test.dart
index 3ed06b75cde1057dd0243aedaea8a5bb1bbd9adc..785ebe7dd44ca5a5757227bd0d1b3a892889c160 100644
--- a/tests/lib/collection/linked_list_test.dart
+++ b/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();