Test ImportElement and fix PrefixElementZ.isTopLevel

pkg/compiler/lib/src/serialization/modelz.dart

 // Copyright (c) 2015, 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.
 
 /// Implementation of the element model used for deserialiation.
 ///
 /// These classes are created by [ElementDeserializer] triggered by the
 /// [Deserializer].
 
 library dart2js.serialization.modelz;
@@ skipping 2268 matching lines @@
     implements PrefixElement {
   bool _isDeferred;
   ImportElement _deferredImport;
   GetterElement _loadLibrary;
 
   PrefixElementZ(ObjectDecoder decoder) : super(decoder);
 
   @override
   accept(ElementVisitor visitor, arg) => visitor.visitPrefixElement(this, arg);
 
+  @override
+  bool get isTopLevel => false;
+
   void _ensureDeferred() {
     if (_isDeferred == null) {
       _isDeferred = _decoder.getBool(Key.IS_DEFERRED);
       if (_isDeferred) {
         _deferredImport = _decoder.getElement(Key.IMPORT);
         _loadLibrary = _decoder.getElement(Key.GETTER);
       }
     }
   }
 
@@ skipping 37 matching lines @@
   }
 
   @override
   Node get node => throw new UnsupportedError('${this}.node');
 
   @override
   bool get hasNode => false;
 
   String toString() => 'MetadataAnnotationZ(${constant.toDartText()})';
 }