fix unsound cast failures in tests

tests/corelib_strong/sort_helper.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.
 
 library sort_helper;
 
 import "package:expect/expect.dart";
 
+typedef Sorter = void Function(List<num>);
+typedef Comparer = int Function(num, num);
+
 class SortHelper {
   SortHelper(this.sortFunction, this.compareFunction) {}
 
   void run() {
     testSortIntLists();
     testSortDoubleLists();
   }
 
-  bool isSorted(List a) {
+  bool isSorted(List<num> 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);
+    var a = new List<int>(40);
 
     for (int i = 0; i < a.length; i++) {
       a[i] = i;
     }
     testSort(a);
 
     for (int i = 0; i < a.length; i++) {
       a[a.length - i - 1] = i;
     }
     testSort(a);
@@ skipping 35 matching lines @@
     for (int i = 21; i < a.length; i++) {
       a[i] = 1;
     }
     a[6] = 2;
     a[13] = 2;
     a[19] = 2;
     a[25] = 2;
     a[33] = 1;
     testSort(a);
 
-    var a2 = new List(0);
+    var a2 = new List<int>(0);
     testSort(a2);
 
-    var a3 = new List(1);
+    var a3 = new List<int>(1);
     a3[0] = 1;
     testSort(a3);
 
     // --------
     // Test insertion sort.
     testInsertionSort(0, 1, 2, 3);
     testInsertionSort(0, 1, 3, 2);
     testInsertionSort(0, 3, 2, 1);
     testInsertionSort(0, 3, 1, 2);
     testInsertionSort(0, 2, 1, 3);
@@ skipping 11 matching lines @@
     testInsertionSort(2, 3, 1, 0);
     testInsertionSort(2, 3, 0, 1);
     testInsertionSort(3, 0, 1, 2);
     testInsertionSort(3, 0, 2, 1);
     testInsertionSort(3, 1, 2, 0);
     testInsertionSort(3, 1, 0, 2);
     testInsertionSort(3, 2, 1, 0);
     testInsertionSort(3, 2, 0, 1);
   }
 
-  void testSort(List a) {
+  void testSort(List<num> a) {
     sortFunction(a);
     Expect.isTrue(isSorted(a));
   }
 
   void testInsertionSort(int i1, int i2, int i3, int i4) {
-    var a = new List(4);
+    var a = new List<int>(4);
     a[0] = i1;
     a[1] = i2;
     a[2] = i3;
     a[3] = i4;
     testSort(a);
   }
 
   void testSortDoubleLists() {
-    List a = new List(40);
+    var a = new List<double>(40);
     for (int i = 0; i < a.length; i++) {
       a[i] = 1.0 * i + 0.5;
     }
     testSort(a);
 
     for (int i = 0; i < a.length; i++) {
       a[i] = 1.0 * (a.length - i) + 0.5;
     }
     testSort(a);
 
     for (int i = 0; i < a.length; i++) {
       a[i] = 1.5;
     }
     testSort(a);
   }
 
-  Function sortFunction;
-  Function compareFunction;
+  Sorter sortFunction;
+  Comparer compareFunction;
 }