Check for importedLibrary is null in importLibraryElement().

I see it in crashes sometimes.

R=brianwilkerson@google.com, pquitslund@google.com

Change-Id: I734d78f0e71c5985c7833fcf6e439fd4c48a5899
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/139601
Reviewed-by: Phil Quitslund <pquitslund@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

pkg/analyzer/lib/dart/element/element.dart

@@ -1048,7 +1048,7 @@ abstract class ExportElement implements Element, UriReferencedElement {
   List<NamespaceCombinator> get combinators;
 
   /// Return the library that is exported from this library by this export
-  /// directive.
+  /// directive, or `null` if the URI has invalid syntax or cannot be resolved.
   LibraryElement get exportedLibrary;
 }
 
@@ -1246,7 +1246,7 @@ abstract class ImportElement implements Element, UriReferencedElement {
   List<NamespaceCombinator> get combinators;
 
   /// Return the library that is imported into this library by this import
-  /// directive.
+  /// directive, or `null` if the URI has invalid syntax or cannot be resolved.
   LibraryElement get importedLibrary;
 
   /// Return `true` if this import is for a deferred library.

pkg/analyzer_plugin/lib/src/utilities/change_builder/change_builder_dart.dart

@@ -1305,7 +1305,8 @@ class DartFileEditBuilderImpl extends FileEditBuilderImpl
     }
 
     for (var import in resolvedUnit.libraryElement.imports) {
-      if (import.importedLibrary.source.uri == uri) {
+      var importedLibrary = import.importedLibrary;
+      if (importedLibrary != null && importedLibrary.source.uri == uri) {
         return ImportLibraryElementResultImpl(import.prefix?.name);
       }
     }

pkg/analyzer_plugin/test/src/utilities/change_builder/dart/import_library_element_test.dart

@@ -70,6 +70,19 @@ import 'package:test/b.dart';
     );
   }
 
+  Future<void> test_withoutPrefix_hasInvalidImport() async {
+    newFile('/home/test/lib/a.dart', content: 'class A {}');
+
+    await _assertImportLibraryElement(
+      initialCode: r'''
+import ':[invalidUri]';
+import 'package:test/a.dart';
+''',
+      uriStr: 'package:test/a.dart',
+      name: 'A',
+    );
+  }
+
   Future<void> test_withoutPrefix_referencedNames_sameElements() async {
     newFile('/home/test/lib/a.dart', content: r'''
 class A {}