Make type system changes backwards compatible

pkg/analyzer/lib/src/task/strong_mode.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 analyzer.src.task.strong_mode;
 
 import 'dart:collection';
 
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/element.dart';
@@ skipping 94 matching lines @@
   /**
    * The classes that have been visited while attempting to infer the types of
    * instance members of some base class.
    */
   HashSet<ClassElementImpl> elementsBeingInferred =
       new HashSet<ClassElementImpl>();
 
   /**
    * Initialize a newly create inferrer.
    */
-  InstanceMemberInferrer(this.typeProvider, this.typeSystem);
+  InstanceMemberInferrer(this.typeProvider, {TypeSystem typeSystem})
+      : typeSystem = (typeSystem != null) ? typeSystem : new TypeSystemImpl();
 
   /**
    * Infer type information for all of the instance members in the given
    * compilation [unit].
    */
   void inferCompilationUnit(CompilationUnitElement unit) {
     inheritanceManager = new InheritanceManager(unit.library);
     unit.types.forEach((ClassElement classElement) {
       try {
         _inferClass(classElement);
@@ skipping 343 matching lines @@
         results.add(element);
       }
     }
   }
 }
 
 /**
  * A class of exception that is not used anywhere else.
  */
 class _CycleException implements Exception {}