Check if assists are requested on a invalid offset.

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

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library test.services.correction.assist;
 
 import 'package:analysis_server/src/protocol.dart';
 import 'package:analysis_server/src/services/correction/assist.dart';
 import 'package:analysis_server/src/services/index/index.dart';
 import 'package:analysis_server/src/services/index/local_memory_index.dart';
@@ skipping 1357 matching lines @@
     assertHasAssistAt(
         'is String',
         AssistKind.INTRODUCE_LOCAL_CAST_TYPE,
         expected);
     assertHasAssistAt(
         'while (p',
         AssistKind.INTRODUCE_LOCAL_CAST_TYPE,
         expected);
   }
 
+  void test_invalidSelection() {
+    _indexTestUnit('');
+    List<Assist> assists = computeAssists(searchEngine, testUnit, -1, 0);
+    expect(assists, isEmpty);
+  }
+
   void test_invertIfStatement_blocks() {
     _indexTestUnit('''
 main() {
   if (true) {
     0;
   } else {
     1;
   }
 }
 ''');
@@ skipping 1233 matching lines @@
   void _indexTestUnit(String code) {
     resolveTestUnit(code);
     index.indexUnit(context, testUnit);
   }
 
   void _setStartEndSelection() {
     offset = findOffset('// start\n') + '// start\n'.length;
     length = findOffset('// end') - offset;
   }
 }