Adds typechecking of return values

tests/corelib/src/SortHelper.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.
 
 class SortHelper {
 
   SortHelper(this.sortFunction, this.compareFunction) {}
 
   void run() {
     testSortIntLists();
     testSortDoubleLists();
   }
 
   void printList(List a) {
     StringBuffer buffer = new StringBuffer();
     for (int i = 0; i < a.length; i++) {
       if (i != 0) buffer.add(",");
       buffer.add(a[i]);
     }
     print("[$buffer]");
   }
 
-  void isSorted(List a) {
+  bool isSorted(List a) {
     for (int i = 1; i < a.length; i++) {
       if (compareFunction(a[i - 1], a[i]) > 0) {
         return false;
       }
     }
     return true;
   }
 
   void testSortIntLists() {
     List a = new List(40);
@@ skipping 119 matching lines @@
 
     for (int i = 0; i < a.length; i++) {
       a[i] = 1.5;
     }
     testSort(a);
   }
 
   Function sortFunction;
   Function compareFunction;
 }