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

pkg/analyzer/lib/src/task/dart.dart

Index: pkg/analyzer/lib/src/task/dart.dart
diff --git a/pkg/analyzer/lib/src/task/dart.dart b/pkg/analyzer/lib/src/task/dart.dart
index 4ff2448fed81fd5929db03745844d9fc6cb73db6..b3ec67ed25bf694ab5c7de1ab0c3640757743243 100644
--- a/pkg/analyzer/lib/src/task/dart.dart
+++ b/pkg/analyzer/lib/src/task/dart.dart
@@ -6545,8 +6545,10 @@ class _SourceClosureTaskInputBuilder implements TaskInputBuilder<List<Source>> {
         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 ||
@@ -6555,8 +6557,10 @@ class _SourceClosureTaskInputBuilder implements TaskInputBuilder<List<Source>> {
         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);
+          }
         }
       }
     }