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

tests/language/loop_exchange3_test.dart

Index: tests/language/loop_exchange3_test.dart
diff --git a/tests/language/loop_exchange3_test.dart b/tests/language/loop_exchange3_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..5b850050a916b9cb4ea837e2a58ebb9c1a95b006
--- /dev/null
+++ b/tests/language/loop_exchange3_test.dart
@@ -0,0 +1,13 @@
+// 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);
+    }
+  }
+}