Convert null-marker in HashMap.forEach.

tests/language/hash_map_null_key_foreach_test.dart

 // Copyright (c) 2013, 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.
 
-// Test that a loop invariant code motion optimization does not try to
-// hoist instructions that may throw.
-
 import "package:expect/expect.dart";
 
-var a = 42;
-var b;
+// Regression test for using `null` as a key with `forEach`.
 
 main() {
-  Expect.throws(() {
-    while (true) {
-      a = 54;
-      b.length;
-    }
+  var x = new Map<int, int>();
+  x[1] = 2;
+  x[null] = 1;
+  int c = 0;
+  x.forEach((int i, int j) {
+    c++;
+    Expect.isTrue(i == null || i is int, 'int or null expected');
   });
-  b = [];
-  Expect.equals(54, a);
+  Expect.equals(2, c);
 }