Emit warning on import of dart:mirrors.

pkg/compiler/lib/src/dart_backend/dart_backend.dart

 // Copyright (c) 2012, 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 dart_backend;
 
 import 'dart:async' show Future;
 import 'dart:math' show max;
 import '../elements/elements.dart';
 import '../dart2jslib.dart';
+import '../library_loader.dart' show LoadedLibraries;
 import '../dart_types.dart';
 import '../tree/tree.dart';
 import '../cps_ir/cps_ir_nodes.dart' as cps_ir;
 import '../constants/expressions.dart';
 import '../cps_ir/optimizers.dart';
 import 'tree_ir_builder.dart' as tree_builder;
 import 'tree_ir_nodes.dart' as tree_ir;
 import '../util/util.dart';
 import '../mirror_renamer/mirror_renamer.dart';
 import 'logical_rewriter.dart' show LogicalRewriter;
@@ skipping 10 matching lines @@
                                          Keyword,
                                          OPEN_PAREN_INFO,
                                          CLOSE_PAREN_INFO,
                                          SEMICOLON_INFO,
                                          IDENTIFIER_INFO;
 
 part 'backend.dart';
 part 'renamer.dart';
 part 'placeholder_collector.dart';
 part 'outputter.dart';