Make sure that we don't assign phis before its uses.

tests/language/loop_exchange3_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.
+
+// This program tripped dart2js.
+main() {
+  int foo;
+  for (var i = 0; i < 10; foo = i, i++) {
+    if(i > 0) {
+      Expect.equals(i - 1, foo);
+    }
+  }
+}