Fix bug with guarded fields and deserialization.

tests/language/vm/optimized_guarded_field_test.dart

Index: tests/language/vm/optimized_guarded_field_test.dart
===================================================================
--- tests/language/vm/optimized_guarded_field_test.dart	(revision 29540)
+++ tests/language/vm/optimized_guarded_field_test.dart	(working copy)
@@ -32,4 +32,23 @@
   a.foo = 123;
   Expect.equals("other", test(a));
   Expect.equals("null", test(b));
+
+  // Test guarded fields with allocation sinking and deoptimization.
+  Expect.equals(43, test_deopt(42, 1));
+  for (var i = 0; i < 20; i++) test_deopt(42, 1);
+  Expect.equals(43, test_deopt(42, 1));
+  Expect.equals("aaabbb", test_deopt("aaa", "bbb"));
 }
+
+
+class C {
+  C(this.x, this.y);
+  final x;
+  final y;
+}
+
+
+test_deopt(a, b) {
+  var c = new C(a, b);
+  return c.x + c.y;
+}