Encapsulate the index based maps for entities.

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/names.dart' show Names;
 import '../elements/entities.dart';
 import '../elements/names.dart';
 import '../elements/types.dart';
@@ skipping 120 matching lines @@
       {bool isExternal,
       bool isConst,
       bool isFromEnvironmentConstructor}) {
     return new JFactoryConstructor(
         memberIndex, enclosingClass, name, parameterStructure,
         isExternal: isExternal,
         isConst: isConst,
         isFromEnvironmentConstructor: isFromEnvironmentConstructor);
   }
 
-  ConstructorBodyEntity createConstructorBody(
+  JConstructorBody createConstructorBody(
       int memberIndex, ConstructorEntity constructor) {
     return new JConstructorBody(memberIndex, constructor);
   }
 
   IndexedFunction createGetter(int memberIndex, LibraryEntity library,
       ClassEntity enclosingClass, Name name, AsyncMarker asyncMarker,
       {bool isStatic, bool isExternal, bool isAbstract}) {
     return new JGetter(memberIndex, library, enclosingClass, name, asyncMarker,
         isStatic: isStatic, isExternal: isExternal, isAbstract: isAbstract);
   }
@@ skipping 414 matching lines @@
   final Entity typeDeclaration;
   final String name;
   final int index;
 
   JTypeVariable(
       this.typeVariableIndex, this.typeDeclaration, this.name, this.index);
 
   String toString() =>
       '${jsElementPrefix}type_variable(${typeDeclaration.name}.$name)';
 }