Improvements for checked mode: check when intializing fields, and when assigning to fields.

tests/language/field_override_test.dart

 // 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.
 // Test overriding of fields.
 
 
 class A {}
 class B1 extends A {}
 class B2 extends A {}
 
@@ skipping 11 matching lines @@
 
 class SubSub extends Super {
   SubSub() : super();
 
   // B2 not assignable to B1
   B2 field;  /// 01: static type warning
 }
 
 main() {
   SubSub val1 = new SubSub();
-  val1.field = new B2(); /// 02: static type warning
+  val1.field = new B2(); /// 02: static type warning, dynamic type error
   Expect.equals(true, val1.field is B2); /// 02: continued
 
   Sub val2 = new Sub();
   val2.field = new A();
   Expect.equals(true, val2.field is A);
   Expect.equals(false, val2.field is B1);
   Expect.equals(false, val2.field is B2);
 
   Super val3 = new Super();
   val3.field = new B1();
   Expect.equals(true, val3.field is B1);
   Expect.equals(false, val3.field is B2);
 }