Support checked mode for field setters.

tests/language/field_type_check2_test.dart

Index: tests/language/field_type_check2_test.dart
===================================================================
--- tests/language/field_type_check2_test.dart	(revision 0)
+++ tests/language/field_type_check2_test.dart	(revision 0)
@@ -0,0 +1,20 @@
+// 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 {
+  A a;
+
+  bar(c) {
+    c.a = 2; /// 01: static type warning, dynamic type error
+  }
+}
+
+class B {
+  int a;
+}
+
+main() {
+  new A().bar(new A()); /// 01: continued
+  new A().bar(new B());
+}