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

tests/lib_2/mirrors/deferred_mirrors_metadata_lib.dart

 library lib;
 
 import "deferred_mirrors_metadata_test.dart";
 @MirrorsUsed(
     targets: const ["main", "main.A", "main.B", "main.C", "main.D", "lib.E"])
 import "dart:mirrors";
 
 class H {
   const H();
 }
@@ skipping 13 matching lines @@
   String c = reflectClass(E).metadata[0].invoke(#toString, []).reflectee;
   String d = reflectClass(E)
       .declarations[#f]
       .metadata[0]
       .invoke(#toString, []).reflectee;
   InstanceMirror i = currentMirrorSystem().findLibrary(#main).metadata[0];
   String a = i.invoke(#toString, []).reflectee;
   String b = i.getField(#b).invoke(#toString, []).reflectee;
   return a + b + c + d;
 }