Fixes and tests for adding library imports.

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.
 
 import 'dart:async';
 
 import 'package:analysis_server/plugin/edit/assist/assist_core.dart';
 import 'package:analysis_server/plugin/edit/assist/assist_dart.dart';
 import 'package:analysis_server/src/plugin/server_plugin.dart';
 import 'package:analysis_server/src/services/correction/assist.dart';
@@ skipping 317 matching lines @@
 main() {
   for (var future in getFutures()) {
   }
 }
 ''');
     await assertHasAssistAt(
         'future in',
         DartAssistKind.ADD_TYPE_ANNOTATION,
         '''
 import 'dart:async';
+
 import 'my_lib.dart';
 main() {
   for (Future<int> future in getFutures()) {
   }
 }
 ''');
   }
 
   test_addTypeAnnotation_declaredIdentifier_OK_final() async {
     await resolveTestUnit('''
@@ skipping 129 matching lines @@
 import 'my_lib.dart';
 main() {
   var v = getFutureInt();
 }
 ''');
     await assertHasAssistAt(
         'v =',
         DartAssistKind.ADD_TYPE_ANNOTATION,
         '''
 import 'dart:async';
+
 import 'my_lib.dart';
 main() {
   Future<int> v = getFutureInt();
 }
 ''');
   }
 
   test_addTypeAnnotation_local_OK_addImport_notLibraryUnit() async {
     // prepare library
     addSource(
@@ skipping 25 matching lines @@
     change = assist.change;
     // verify
     {
       var testFileEdit = change.getFileEdit('/app.dart');
       var resultCode = SourceEdit.applySequence(appCode, testFileEdit.edits);
       expect(
           resultCode,
           '''
 library my_app;
 import 'dart:async';
+
 import 'my_lib.dart';
 part 'test.dart';
 ''');
     }
     {
       var testFileEdit = change.getFileEdit('/test.dart');
       var resultCode = SourceEdit.applySequence(testCode, testFileEdit.edits);
       expect(
           resultCode,
           '''
@@ skipping 4214 matching lines @@
 
   @override
   final AstProvider astProvider;
 
   @override
   final CompilationUnit unit;
 
   _DartAssistContextForValues(this.source, this.selectionOffset,
       this.selectionLength, this.analysisDriver, this.astProvider, this.unit);
 }