Silence analyzer warnings in array_bounds_check_generalization_test.

tests/standalone/array_bounds_check_generalization_test.dart

 // Copyright (c) 2012, 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=--optimization_counter_threshold=10 --no-use-osr
 
 import "package:expect/expect.dart";
 
 test1(a, start, step, N) {
   var e;
@@ skipping 15 matching lines @@
   var e;
   for (var i = 0, j = 1, k = 0; i < a.length - 1; i++, j++, k++) {
     e = b[k] = a[j - 1];
   }
   return e;
 }
 
 test4(a, b) {
   var e;
   if (a.length < 2) {
-    return;
+    return null;
   }
 
   for (var i = 0, j = 1, k = 0; i < a.length - 1; i++, j++, k++) {
     e = b[k] = a[j - 1];
   }
   return e;
 }
 
 test5(a, b, k0) {
   var e;
   if (a.length < 2) {
-    return;
+    return null;
   }
 
   if (k0 > 1) {
-    return;
+    return null;
   }
 
   for (var i = 0, j = 1, k = 0; i < a.length - 1; i++, j++, k++) {
     e = b[k - k0] = a[j - 1];
   }
   return e;
 }
 
 test6(a, M, N) {
   var e = 0;
@@ skipping 15 matching lines @@
     Expect.equals(a[a.length - 2], test4(a, b));
     Expect.equals(a[a.length - 2], test5(a, b, 0));
     Expect.equals(6 , test6(a, 2, 2));
   }
 
   test1(a, 0, 2, a.length ~/ 2);
   Expect.throws(() => test1(a, 1, 1, a.length));
   Expect.throws(() => test2(a, new List(a.length - 1)));
   Expect.throws(() => test6(a, 4, 3));
 }