Silence analyzer warnings in array_bounds_check_generalization_test.

TBR=fschneider@google.com

Review URL: https://codereview.chromium.org//631143003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@40973 260f80e4-7a28-3924-810f-c04153c831b5

tests/standalone/array_bounds_check_generalization_test.dart

@@ -33,7 +33,7 @@ test3(a, b) {
 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++) {
@@ -45,11 +45,11 @@ test4(a, b) {
 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++) {