Update comments in core lib showing proper factory syntax.

tests/language/src/Factory3Test.dart

+// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+// VMOptions=--enable_type_checks --enable_asserts
+
+interface Link<T> extends Iterable<T> factory LinkFactory {
+  Link(T head, [Link<T> tail]);
+  Link<T> prepend(T element);
+}
+
+interface EmptyLink<T> extends Link<T> factory LinkTail<T> {
+  const EmptyLink();
+}
+
+class LinkFactory {
+  factory Link<T>(head, [Link tail]) {
+    return new LinkEntry<T>(head, tail);
+  }
+}
+
+class AbstractLink<T> implements Link<T> {
+  const AbstractLink();
+  Link<T> prepend(T element) {
+    print("$element");
+    if (0 is T) {
+      throw "0 is not a T";
+    }
+    return new Link<T>(element, this);
+  }
+}
+
+class LinkTail<T> extends AbstractLink<T> implements EmptyLink<T> {
+  const LinkTail();
+}
+
+class LinkEntry<T> extends AbstractLink<T> {
+  LinkEntry(T head, Link<T> realTail);
+}
+
+class Fisk {
+  Link<Fisk> nodes = const EmptyLink<Fisk>();
+  final int id;
+  Fisk(this.id);
+  toString() => id.toString();
+}
+
+main() {
+  new Fisk(0).nodes.prepend(new Fisk(1)).prepend(new Fisk(2));
+}
+