Fix tests/language/cast2_test.dart to eliminate static warnings.

tests/language/cast2_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.
 // Dart test program for constructors and initializers.
 
 import "package:expect/expect.dart";
 
 // Test 'expression as Type' casts.
 
 class C {
   final int foo = 42;
 
   int val = 0;
   void inc() { ++val; }
 }
 
 class D extends C {
   final int bar = 37;
 }
 
 main() {
-  Object oc = new C();
-  Object od = new D();
+  C oc = new C();
+  D od = new D();
 
   (oc as dynamic).bar;  /// 01: runtime error
 
   // Casts should always evaluate the left-hand side, if only for its effects.
   oc.inc() as dynamic;
   Expect.equals(1, oc.val);
   oc.inc() as Object;
   Expect.equals(2, oc.val);
 }