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

tests/language/loop_exchange_test.dart

Index: tests/language/loop_exchange_test.dart
diff --git a/tests/language/loop_exchange_test.dart b/tests/language/loop_exchange_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..9b06a177896ee6dcaf84043ab95f1dd3da12c1e6
--- /dev/null
+++ b/tests/language/loop_exchange_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 x = 1;
+  var y = 2;
+  for (int i = 0; i < 2; i++) {
+    if (i == 1) Expect.equals(2, x);
+    var tmp = x;
+    x = y;
+    y = tmp;
+  }
+}
+