We must also check if the first input of a phi is unknown.

tests/language/src/TypePropagationInForUpdateTest.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.
+// Check that phi type computation in the Dart2Js compiler does the
+// correct thing.
+
+bar() => 'foo';
+
+main() {
+  Expect.throws(foo1, (e) => e is IllegalArgumentException);
+  Expect.throws(foo2, (e) => e is IllegalArgumentException);
+}
+
+foo1() {
+  var a = bar();
+  for (;; a = 1 + a) {
+    if (a != 'foo') return;
+  }
+}
+
+foo2() {
+  var a = bar();
+  for (;; a = 1 + a) {
+    if (a != 'foo') break;
+  }
+}