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

tests/language/loop_exchange4_test.dart

Index: tests/language/loop_exchange4_test.dart
diff --git a/tests/language/loop_exchange4_test.dart b/tests/language/loop_exchange4_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..9c1f459ca85c97df33d1680f15f02259a74801b9
--- /dev/null
+++ b/tests/language/loop_exchange4_test.dart
@@ -0,0 +1,16 @@
+// 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() {
+  var count = 0;
+  for (var i = 0; i < 1; i++, count += i) {}
+
+  int foo;
+  for (var i = 0; i < 10; foo = i, i++) {
+    if(i > 0) {
+      Expect.equals(i - 1, foo);
+    }
+  }
+}