Added back helper file accidentally deleted during test block 219 migration.

tests/lib_2/mirrors/deferred_mirrors_update_test.dart

 library main;
 
 // Test that the library-mirrors are updated after loading a deferred library.
 
 @MirrorsUsed(targets: "D")
 import "dart:mirrors";
 
 import "deferred_mirrors_update_lib.dart" deferred as l;
 
 class D {}
 
 void main() {
   print(reflectClass(D).owner);
   l.loadLibrary().then((_) {
     l.foo();
   });
 }