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

pkg/analyzer/lib/src/dart/element/builder.dart

Index: pkg/analyzer/lib/src/dart/element/builder.dart
diff --git a/pkg/analyzer/lib/src/dart/element/builder.dart b/pkg/analyzer/lib/src/dart/element/builder.dart
index 39311d18b5e289c2d9b1724e44cb292965feb92c..48a732f47f28750b162918c7ed26b80c17cff89c 100644
--- a/pkg/analyzer/lib/src/dart/element/builder.dart
+++ b/pkg/analyzer/lib/src/dart/element/builder.dart
@@ -160,6 +160,8 @@ class DirectiveElementBuilder extends SimpleAstVisitor<Object> {
 
   @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
@@ -200,6 +202,9 @@ class DirectiveElementBuilder extends SimpleAstVisitor<Object> {
 
   @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;