move TypeSystem classes to their own file

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';
-import 'package:analyzer/src/generated/resolver.dart';
+import 'package:analyzer/src/generated/resolver.dart'
+    show TypeProvider, InheritanceManager;
+import 'package:analyzer/src/generated/type_system.dart';
 import 'package:analyzer/src/generated/utilities_dart.dart';
 
 /**
  * Set the type of the sole parameter of the given [element] to the given [type].
  */
 void setParameterType(PropertyAccessorElement element, DartType type) {
   if (element is PropertyAccessorElementImpl) {
     ParameterElement parameter = _getParameter(element);
     if (parameter is ParameterElementImpl) {
       //
@@ skipping 475 matching lines @@
         results.add(element);
       }
     }
   }
 }
 
 /**
  * A class of exception that is not used anywhere else.
  */
 class _CycleException implements Exception {}