Fix exception when libraries cannot be resolved (issue 36863)

Change-Id: I293cbcc2ba966a62bb40b1eba9c3d25b524cb8cb
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/101481
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>
diff --git a/pkg/analyzer_plugin/lib/src/utilities/change_builder/dart/import_library_element.dart b/pkg/analyzer_plugin/lib/src/utilities/change_builder/dart/import_library_element.dart
index f20a786..f135dd8 100644
--- a/pkg/analyzer_plugin/lib/src/utilities/change_builder/dart/import_library_element.dart
+++ b/pkg/analyzer_plugin/lib/src/utilities/change_builder/dart/import_library_element.dart
@@ -112,7 +112,7 @@
 
   // Check for existing imports of the requested library.
   for (var import in targetLibrary.imports) {
-    if (import.importedLibrary.source.uri == requestedLibraryUri) {
+    if (import.importedLibrary?.source?.uri == requestedLibraryUri) {
       var importedNames = import.namespace.definedNames;
       if (import.prefix == null) {
         if (canUseUnprefixedImport &&