Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/syntax_error_test.dart

 // Copyright (c) 2013, 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.
 
 // Regression test for Issue 15744
 // Also, tests that syntax errors in reflected classes are reported correctly.
 
+library lib;
+
+@MirrorsUsed(targets: "lib")
 import 'dart:mirrors';
 
 class MD {
   final String name;
   const MD({this.name});
 }
 
 @MD(name:'A')
 class A {}
 
 @MD(name:'B')
 class B {
   static x = { 0: 0; };  /// 01: compile-time error
 }
 
 main() {
   reflectClass(A).metadata;
   reflectClass(B).newInstance(const Symbol(''), []);
 }