Use LibraryIndex in CoreTypes instead of extending it.

pkg/kernel/lib/transformations/treeshaker.dart

 // Copyright (c) 2016, 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 kernel.tree_shaker;
 
 import '../ast.dart';
 import '../class_hierarchy.dart';
 import '../core_types.dart';
 import '../type_environment.dart';
@@ skipping 204 matching lines @@
         this._usedMembersWithHost =
             new List<Set<Member>>(hierarchy.classes.length),
         this._classRetention = new List<ClassRetention>.filled(
             hierarchy.classes.length, ClassRetention.None) {
     _visitor = new _TreeShakerVisitor(this);
     _covariantVisitor = new _ExternalTypeVisitor(this, isCovariant: true);
     _contravariantVisitor =
         new _ExternalTypeVisitor(this, isContravariant: true);
     _invariantVisitor = new _ExternalTypeVisitor(this,
         isCovariant: true, isContravariant: true);
-    _mirrorsLibrary = coreTypes.tryGetLibrary('dart:mirrors');
+    _mirrorsLibrary = coreTypes.mirrorsLibrary;
     try {
       _build();
     } on _UsingMirrorsException {
       isUsingMirrors = true;
     }
   }
 
   void _build() {
     if (program.mainMethod == null) {
       throw 'Cannot perform tree shaking on a program without a main method';
@@ skipping 846 matching lines @@
         classNode == coreTypes.futureClass ||
         classNode == coreTypes.streamClass ||
         classNode == coreTypes.listClass ||
         classNode == coreTypes.mapClass;
   }
 }
 
 /// Exception that is thrown to stop the tree shaking analysis when a use
 /// of `dart:mirrors` is found.
 class _UsingMirrorsException {}