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

tests/lib_strong/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();
-}
-
-class F {
-  @H()
-  final int f;
-}
-
-@C()
-class E {
-  @D()
-  var f;
-}
-
-String foo() {
-  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;
-}