Fixes and tests for adding library imports.

pkg/analysis_server/test/services/correction/fix_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/fix/fix_core.dart';
 import 'package:analysis_server/plugin/edit/fix/fix_dart.dart';
 import 'package:analysis_server/src/services/correction/fix.dart';
 import 'package:analysis_server/src/services/correction/fix_internal.dart';
@@ skipping 4694 matching lines @@
     await resolveTestUnit('''
 import 'lib.dart';
 main() {
   test(getFuture());
 }
 ''');
     await assertHasFix(
         DartFixKind.CREATE_FUNCTION,
         '''
 import 'dart:async';
+
 import 'lib.dart';
 main() {
   test(getFuture());
 }
 
 void test(Future future) {
 }
 ''');
   }
 
@@ skipping 2176 matching lines @@
   @override
   final AnalysisError error;
 
   _DartFixContextImpl(this.resourceProvider, this.analysisDriver,
       this.astProvider, this.unit, this.error);
 
   @override
   GetTopLevelDeclarations get getTopLevelDeclarations =>
       analysisDriver.getTopLevelNameDeclarations;
 }