Split ClassEnv into env and data to avoid copying types/constants from the k-model

pkg/compiler/lib/src/js_model/elements.dart

 // Copyright (c) 2017, 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 dart2js.js_model.elements;
 
 import '../common_elements.dart';
 import '../constants/constant_system.dart';
 import '../elements/elements.dart';
 import '../elements/entities.dart';
@@ skipping 91 matching lines @@
     return new JLibrary(libraryIndex, name, canonicalUri);
   }
 
   IndexedClass createClass(LibraryEntity library, int classIndex, String name,
       {bool isAbstract}) {
     return new JClass(library, classIndex, name, isAbstract: isAbstract);
   }
 
   TypeVariableEntity createTypeVariable(
       Entity typeDeclaration, String name, int index) {
-    throw new UnsupportedError('JsElementCreatorMixin.createTypeVariable');
+    return new JTypeVariable(typeDeclaration, name, index);
   }
 
   IndexedConstructor createGenerativeConstructor(
       int memberIndex,
       ClassEntity enclosingClass,
       Name name,
       ParameterStructure parameterStructure,
       {bool isExternal,
       bool isConst}) {
     return new JGenerativeConstructor(
@@ skipping 650 matching lines @@
 
   @override
   bool get isFunctionApplyUsed => _backendUsage.isFunctionApplyUsed;
 
   @override
   bool get isMirrorsUsed => _backendUsage.isMirrorsUsed;
 
   @override
   bool get isNoSuchMethodUsed => _backendUsage.isNoSuchMethodUsed;
 }