Fix usage of UNKNOWN/CONFLICTING types for union.

tests/language/field_optimization_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.
+
+// Dart test program to test type-based optimization on fields.
+
+
+class A {
+  var x;
+  A() : x = 0;
+  foo() {
+    x++;
+  }
+  toto() {
+    x = 99;
+  }
+  bar(y) {
+    x = y;
+  }
+}
+
+class B {
+  operator +(other) => "ok";
+}
+main() {
+  var a = new A();
+  a.foo();
+  a.toto();
+  a.bar("str");
+  a.bar(new B());
+  a.foo();
+  Expect.equals("ok", a.x);
+}