Support checked mode for field setters.

tests/language/field_type_check_test.dart

Index: tests/language/field_type_check_test.dart
===================================================================
--- tests/language/field_type_check_test.dart	(revision 0)
+++ tests/language/field_type_check_test.dart	(revision 0)
@@ -0,0 +1,11 @@
+// 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 A {
+  int e;
+}
+
+int main() {
+  new A().e = "String"; /// 01: static type warning, dynamic type error
+}