Move dart2jslib parts into separate libraries.

pkg/compiler/lib/src/elements/modelx.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.
 
 library elements.modelx;
 
 import 'common.dart';
 import 'elements.dart';
 import '../constants/constant_constructors.dart';
 import '../constants/constructors.dart';
 import '../constants/expressions.dart';
+import '../diagnostic_listener.dart';
 import '../helpers/helpers.dart';
+import '../messages.dart';
+import '../script.dart';
 import '../tree/tree.dart';
 import '../util/util.dart';
 import '../resolution/resolution.dart';
 import '../resolution/class_members.dart' show ClassMemberMixin;
 
 import '../dart2jslib.dart' show
     Backend,
     Compiler,
     Constant,
     DartType,
@@ skipping 2880 matching lines @@
   AstElement get definingElement;
 
   bool get hasResolvedAst => definingElement.hasTreeElements;
 
   ResolvedAst get resolvedAst {
     return new ResolvedAst(declaration,
         definingElement.node, definingElement.treeElements);
   }
 
 }