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

pkg/analyzer/lib/src/task/dart.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 analyzer.src.task.dart;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
@@ skipping 6527 matching lines @@
   @override
   void set currentValue(Object value) {
     LibraryElement library = value;
     if (_libraries.add(library)) {
       if (kind == _SourceClosureKind.IMPORT ||
           kind == _SourceClosureKind.IMPORT_EXPORT) {
         List<ImportElement> imports = library.imports;
         int length = imports.length;
         for (int i = 0; i < length; i++) {
           ImportElement importElement = imports[i];
-          Source importedSource = importElement.importedLibrary.source;
-          _newSources.add(importedSource);
+          Source importedSource = importElement.importedLibrary?.source;
+          if (importedSource != null) {
+            _newSources.add(importedSource);
+          }
         }
       }
       if (kind == _SourceClosureKind.EXPORT ||
           kind == _SourceClosureKind.IMPORT_EXPORT) {
         List<ExportElement> exports = library.exports;
         int length = exports.length;
         for (int i = 0; i < length; i++) {
           ExportElement exportElement = exports[i];
-          Source exportedSource = exportElement.exportedLibrary.source;
-          _newSources.add(exportedSource);
+          Source exportedSource = exportElement.exportedLibrary?.source;
+          if (exportedSource != null) {
+            _newSources.add(exportedSource);
+          }
         }
       }
     }
   }
 
   @override
   bool get flushOnAccess => false;
 
   @override
   List<Source> get inputValue {
     return _libraries.map((LibraryElement library) => library.source).toList();
   }
 
   @override
   void currentValueNotAvailable() {
     // Nothing needs to be done.  moveNext() will simply go on to the next new
     // source.
   }
 
   @override
   bool moveNext() {
     if (_newSources.isEmpty) {
       return false;
     }
     currentTarget = _newSources.removeLast();
     return true;
   }
 }