Suppress follow-on errors when a file is imported with either a prefix or a show clause

pkg/analysis_server/lib/src/services/completion/dart/completion_manager.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.
 
 library services.completion.dart.manager;
 
 import 'dart:async';
 
 import 'package:analysis_server/plugin/protocol/protocol.dart';
 import 'package:analysis_server/src/provisional/completion/completion_core.dart'
@@ skipping 268 matching lines @@
     checkAborted();
     if (_resolvedImports != null) {
       return _resolvedImports;
     }
     LibraryElement libElem = libraryElement;
     if (libElem == null) {
       return null;
     }
     _resolvedImports = <ImportElement>[];
     for (ImportElement importElem in libElem.imports) {
-      if (importElem.importedLibrary.exportNamespace == null) {
+      if (importElem.importedLibrary?.exportNamespace == null) {
         await _computeAsync(this, importElem.importedLibrary.source,
             LIBRARY_ELEMENT4, performance, 'resolve imported library');
         checkAborted();
       }
       _resolvedImports.add(importElem);
     }
     return _resolvedImports;
   }
 
   @override
@@ skipping 193 matching lines @@
           if (start <= requestOffset && requestOffset <= end) {
             // Replacement range for import URI
             return new ReplacementRange(start, end - start);
           }
         }
       }
     }
     return new ReplacementRange(requestOffset, 0);
   }
 }