VM: Better type propagation for string objects.

tests/language/vm/optimized_identical_test.dart

 // Copyright (c) 2013, 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.
 // Test various optimizations and deoptimizations of optimizing compiler..
 // VMOptions=--optimization-counter-threshold=10 --no-constant-propagation
 
 import "package:expect/expect.dart";
 
 // Test canonicalization of identical with double input.
 // Constant propagation is disabled so that canonicalization is run
 // one time less than usual.
 
 test(a) {
   var dbl = a + 1.0;
   if (!identical(dbl, true)) {
     return "ok"; 
   }
   throw "fail";
 }
 
+Object Y = 1.0;
+
+test_object_type(x) => identical(x, Y);
+
 main() {
   for (var i = 0; i < 20; i++) test(0);
   Expect.equals("ok", test(0));
+
+  var x = 0.0 + 1.0;
+  for (var i = 0; i < 20; i++) test_object_type(x);
+  Expect.equals(true, test_object_type(x));
 }