Fix a bug where a block would be visited twice: by a non-dominating loop, and by its dominator.

tests/language/src/NonDominatingLoopTest.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.
+
+int calls = 0;
+
+void callMeOnce() {
+  Expect.equals(0, calls);
+  calls++;
+}
+
+main() {
+  int i = 0;
+  do {
+    i++;
+    if (i > 3) break;
+  } while (i < 10);
+
+  callMeOnce();
+  Expect.equals(4, i);
+}