Clear the element for imports and exports when the target doesn't exist.

pkg/analyzer/lib/src/dart/element/builder.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.dart.element.builder;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
@@ skipping 142 matching lines @@
     //
     // Populate the library element.
     //
     libraryElement.imports = imports;
     libraryElement.exports = exports;
     return null;
   }
 
   @override
   Object visitExportDirective(ExportDirective node) {
+    // Remove previous element. (It will remain null if the target is missing.)
+    node.element = null;

[Brian Wilkerson, 2016/02/27 18:05:21]

I don't understand why this would be necessary. DirectiveElementBuilder is only
used in the BuildDirectiveElementsTask, and at the point at which that task is
called the (Import|Export)Directive should not have an element associated with
it.

If we're getting a (partially) resolved AST when that task is being performed,
then we have probably failed much earlier when we ought to have invalidated the
AST.

My concern is that this change would only mask a different bug.

     Source exportedSource = node.source;
     if (exportedSource != null && context.exists(exportedSource)) {
       // The exported source will be null if the URI in the export
       // directive was invalid.
       LibraryElement exportedLibrary = exportLibraryMap[exportedSource];
       if (exportedLibrary != null) {
         ExportElementImpl exportElement = new ExportElementImpl(node.offset);
         exportElement.metadata = _getElementAnnotations(node.metadata);
         StringLiteral uriLiteral = node.uri;
         if (uriLiteral != null) {
@@ skipping 20 matching lines @@
               CompileTimeErrorCode.EXPORT_OF_NON_LIBRARY,
               [uriLiteral.toSource()]));
         }
       }
     }
     return null;
   }
 
   @override
   Object visitImportDirective(ImportDirective node) {
+    // Remove previous element. (It will remain null if the target is missing.)
+    node.element = null;
+
     String uriContent = node.uriContent;
     if (DartUriResolver.isDartExtUri(uriContent)) {
       libraryElement.hasExtUri = true;
     }
     Source importedSource = node.source;
     if (importedSource != null && context.exists(importedSource)) {
       // The imported source will be null if the URI in the import
       // directive was invalid.
       LibraryElement importedLibrary = importLibraryMap[importedSource];
       if (importedLibrary != null) {
@@ skipping 1200 matching lines @@
     return null;
   }
 
   /**
    * Return the lexical identifiers associated with the given [identifiers].
    */
   static List<String> _getIdentifiers(NodeList<SimpleIdentifier> identifiers) {
     return identifiers.map((identifier) => identifier.name).toList();
   }
 }