dart2dart: Fix fallthrough in while statements.

tests/language/rewrite_while_test.dart

+// Copyright (c) 2014, 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.
+
+import "package:expect/expect.dart";
+
+baz() {}
+
+loop1(x) {
+    var n = 0;
+    while (n < x) {
+        n = n + 1;
+    }
+    return n;
+}
+
+loop2(x) {
+    var n = 0;
+    if (x < 100) {
+        while (n < x) {
+            n = n + 1;
+        }
+    }
+    baz();
+    return n;
+}
+
+main() {
+    Expect.equals(10, loop1(10));
+    Expect.equals(10, loop2(10));
+    Expect.equals(0, loop2(200));
+}