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)
@@ -17,6 +17,41 @@
 
 test(obj) => obj.foo == null ? "null" : "other";
 
+
+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;
+}
+
+
+create_error (x) {
+  return x as int;
+}
+
+check_stacktrace(e) {
+  var s = e.stackTrace;
+  if (identical(s, null)) throw "FAIL";
+  // s should never be null.
+  return "OK";
+}
+
+test_stacktrace() {
+  try {
+    create_error("bar");
+  } catch (e) {
+    Expect.equals("OK", check_stacktrace(e));
+    for (var i=0; i<20; i++) check_stacktrace(e);
+    Expect.equals("OK", check_stacktrace(e));
+  }
+}
+
+
 main() {
   var a = new A();
   var b = new B();
@@ -32,4 +67,13 @@
   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"));
+
+  // Regression test for fields initialized in native code (Error._stackTrace).
+  test_stacktrace();
 }