Fix usage of UNKNOWN/CONFLICTING types for union.

tests/language/field_optimization2_test.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.
 
-class B {
-  foo(i) => 499 + i;
+// Dart test program to test type-based optimization on fields.
+
+
+class A {
+  var x;
+  A() : x = new B();
+  foo() {
+    x++;
+  }
 }
 
-class A {
-  var b;
-  A() : b = new B();
-
-  foo(i) { return (() => b.foo(i))(); }
+class B {
+  operator +(other) => 498;
 }
-
 main() {
   var a = new A();
-  Expect.equals(510, a.foo(11));
-  var f = a.foo;
-  Expect.equals(521, f(22));
+  a.foo();
+  a.foo();
+  Expect.equals(499, a.x);
 }
-