Fix algorithm generics.

lib/src/utils.dart

 // Copyright (c) 2014, 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.
 
 /// A pair of values.
 class Pair<E, F> {
   E first;
   F last;
 
   Pair(this.first, this.last);
 }
+
+/// Returns a [Comparator] that asserts that its first argument is comparable.
+Comparator/*<T>*/ defaultCompare/*<T>*/() =>
+    (value1, value2) => (value1 as Comparable).compareTo(value2);