Don't recreate CoreTypes in transformers. Pass it in.

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';
 import '../library_index.dart';
 
-Program transformProgram(Program program, {List<ProgramRoot> programRoots}) {
-  new TreeShaker(program, programRoots: programRoots).transform(program);
+Program transformProgram(CoreTypes coreTypes, Program program,
+    {List<ProgramRoot> programRoots}) {
+  new TreeShaker(coreTypes, program, programRoots: programRoots)
+      .transform(program);
   return program;
 }
 
 enum ProgramRootKind {
   /// The root is a class which will be instantiated by
   /// external / non-Dart code.
   ExternallyInstantiatedClass,
 
   /// The root is a setter function or a field.
   Setter,
@@ skipping 135 matching lines @@
 
   Library _mirrorsLibrary;
 
   /// Set to true if any use of the `dart:mirrors` API is found.
   bool isUsingMirrors = false;
 
   /// If we have roots, we will shake, even if we encounter some elements from
   /// the mirrors library.
   bool get forceShaking => programRoots != null && programRoots.isNotEmpty;
 
-  TreeShaker(Program program,
+  TreeShaker(CoreTypes coreTypes, Program program,
       {ClassHierarchy hierarchy,
-      CoreTypes coreTypes,
       bool strongMode: false,
       List<ProgramRoot> programRoots})
       : this._internal(program, hierarchy ?? new ClassHierarchy(program),
-            coreTypes ?? new CoreTypes(program), strongMode, programRoots);
+            coreTypes, strongMode, programRoots);
 
   bool isMemberBodyUsed(Member member) {
     return _usedMembers.containsKey(member);
   }
 
   bool isMemberOverridden(Member member) {
     return _overriddenMembers.contains(member);
   }
 
   bool isMemberUsed(Member member) {
@@ skipping 895 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 {}