Check if assists are requested on a invalid offset.

pkg/analysis_server/test/services/correction/assist_test.dart

Index: pkg/analysis_server/test/services/correction/assist_test.dart
diff --git a/pkg/analysis_server/test/services/correction/assist_test.dart b/pkg/analysis_server/test/services/correction/assist_test.dart
index 793c8708aef6b9cd12b0eba051715e4ffbd17d76..b89c6fef7826363d16c8c100a370cb007d61669d 100644
--- a/pkg/analysis_server/test/services/correction/assist_test.dart
+++ b/pkg/analysis_server/test/services/correction/assist_test.dart
@@ -1375,6 +1375,12 @@ main(p) {
         expected);
   }
 
+  void test_invalidSelection() {
+    _indexTestUnit('');
+    List<Assist> assists = computeAssists(searchEngine, testUnit, -1, 0);
+    expect(assists, isEmpty);
+  }
+
   void test_invertIfStatement_blocks() {
     _indexTestUnit('''
 main() {