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

tests/language/super_closure_test.dart

 // Copyright (c) 2011, 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.
 
 // Dart test program for testing access to super from closure.
 
 class Super {
-  int superX = "super";
+  var superX = "super";
   get x => superX;
 }
 
 class Sub extends Super {
   var subX = "sub";
   get x => subX;
 
   buildClosures() => [() => x, () => this.x, () => super.x];
 }
 
 main() {
   var closures = new Sub().buildClosures();
   Expect.equals(3, closures.length);
   Expect.equals("sub", closures[0]());
   Expect.equals("sub", closures[1]());
   Expect.equals("super", closures[2]());
 }