Add prefix constraints for deferred imports.

tests/language/deferred_no_prefix_negative_test.dart

 // Copyright (c) 2014, 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 {
-  static var a;
-  var b;
-}
+import "dart:async";
 
-class B extends A {
+// Loading a deferred library without prefix is not allowed.
+@lazy import "deferred_constraints_lib2.dart";
 
-}
+const lazy = const DeferredLibrary('lib');
 
-class C extends B {
-  var a; /// 01: static type warning
-  static var b; /// 02: compile-time error
-}
-
-void main() {
-  new C();
-}
+void main() {}