Add mirrors tests

test/codegen/lib/mirrors/load_library_test.dart

+// Copyright (c) 2015, 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.
+
+library load_library;
+
+import 'dart:mirrors';
+import 'package:expect/expect.dart';
+import 'package:async_helper/async_helper.dart';
+
+import 'other_library.dart' deferred as other;
+
+main() {
+  var ms = currentMirrorSystem();
+  LibraryMirror thisLibrary = ms.findLibrary(#load_library);
+  var dep =
+      thisLibrary.libraryDependencies.singleWhere((d) => d.prefix == #other);
+  Expect.isNull(dep.targetLibrary, "should not be loaded yet");
+
+  asyncStart();
+  dep.loadLibrary().then((_) {
+    asyncEnd();
+    Expect.isNotNull(dep.targetLibrary);
+    Expect.equals(#test.other_library, dep.targetLibrary.simpleName);
+    Expect.equals(42, other.topLevelMethod());
+  });
+}