Add missing checks for null imported/exportedLibraries.

pkg/analyzer/lib/src/generated/resolver.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 analyzer.src.generated.resolver;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/standard_resolution_map.dart';
@@ skipping 2086 matching lines @@
   }
 
   @override
   Object visitExportDirective(ExportDirective node) {
     ExportElement exportElement = node.element;
     if (exportElement != null) {
       // The element is null when the URI is invalid
       LibraryElement library = exportElement.exportedLibrary;
       if (library != null && !library.isSynthetic) {
         for (Combinator combinator in node.combinators) {
-          _checkCombinator(exportElement.exportedLibrary, combinator);
+          _checkCombinator(library, combinator);
         }
       }
     }
     return super.visitExportDirective(node);
   }
 
   @override
   Object visitIfStatement(IfStatement node) {
     Expression conditionExpression = node.condition;
     conditionExpression?.accept(this);
@@ skipping 1957 matching lines @@
         }
       }
     }
   }
 
   /**
    * Recursively add any exported library elements into the [libraryMap].
    */
   void _addAdditionalLibrariesForExports(LibraryElement library,
       ImportDirective importDirective, Set<LibraryElement> visitedLibraries) {
-    if (!visitedLibraries.add(library)) {
+    if (library == null || !visitedLibraries.add(library)) {
       return;
     }
     List<ExportElement> exports = library.exports;
     int length = exports.length;
     for (int i = 0; i < length; i++) {
       ExportElement exportElt = exports[i];
       LibraryElement exportedLibrary = exportElt.exportedLibrary;
       _putIntoLibraryMap(exportedLibrary, importDirective);
       _addAdditionalLibrariesForExports(
           exportedLibrary, importDirective, visitedLibraries);
@@ skipping 6831 matching lines @@
       return null;
     }
     if (identical(node.staticElement, variable)) {
       if (node.inSetterContext()) {
         result = true;
       }
     }
     return null;
   }
 }