Postpone the argument type optimizations until the types are stable.

tests/language/propagated_argument_type_check_test.dart

Index: tests/language/propagated_argument_type_check_test.dart
diff --git a/tests/language/propagated_argument_type_check_test.dart b/tests/language/propagated_argument_type_check_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..f4bb713ed9fe04634cff017b0195d66822961266
--- /dev/null
+++ b/tests/language/propagated_argument_type_check_test.dart
@@ -0,0 +1,25 @@
+// 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.
+
+main() {
+  Expect.equals("str", foo("str"));
+}
+
+foo(y) {
+  var x = 3;
+  for (int i = 0; i < 2; i++) {
+    // Make sure that we don't think that the type of x is necessarily
+    // a number and optimize the x + y expression based on that. The
+    // value of x changes later...
+    if (i == 1) return bar(x + y);
+    x = new A();
+  }
+}
+
+bar(t) => t;
+
+class A {
+  A() {}
+  operator +(x) => x;
+}