Move dart2js from sdk/lib/_internal/compiler to pkg/compiler

pkg/compiler/lib/src/js_backend/type_variable_handler.dart

 // Copyright (c) 2013, 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.
 
 part of js_backend;
 
 /**
  * Handles construction of TypeVariable constants needed at runtime.
  */
 class TypeVariableHandler {
@@ skipping 160 matching lines @@
   }
 
   List<int> typeVariablesOf(ClassElement classElement) {
     List<int> result = typeVariables[classElement];
     if (result == null) {
       result = const <int>[];
     }
     return result;
   }
 }