Add ImportElement and ExportElement

pkg/compiler/lib/src/constants/expressions.dart

 // Copyright (c) 2014, 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.constants.expressions;
 
 import '../constants/constant_system.dart';
 import '../core_types.dart';
 import '../dart_types.dart';
 import '../diagnostics/invariant.dart' show
@@ skipping 1545 matching lines @@
   }
 
   @override
   void visitNamed(NamedArgumentReference exp, [_]) {
     // TODO(johnniwinther): Maybe this should throw.
     sb.write('args[${exp.name}]');
   }
 
   @override
   void visitDeferred(DeferredConstantExpression exp, context) {
-    sb.write(exp.prefix.deferredImport.prefix.source);
+    sb.write(exp.prefix.name);
     sb.write('.');
     write(exp, exp.expression);
   }
 
   @override
   void visitBoolFromEnvironment(BoolFromEnvironmentConstantExpression exp,
                                 [_]) {
     sb.write('const bool.fromEnvironment(');
     visit(exp.name);
     if (exp.defaultValue != null) {
@@ skipping 21 matching lines @@
     visit(exp.name);
     if (exp.defaultValue != null) {
       sb.write(', defaultValue: ');
       visit(exp.defaultValue);
     }
     sb.write(')');
   }
 
   String toString() => sb.toString();
 }