Update edit.importElements response data to support part files

pkg/analysis_server/test/integration/edit/import_elements_test.dart

 // Copyright (c) 2017, 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/protocol/protocol_generated.dart';
 import 'package:analyzer_plugin/protocol/protocol_common.dart';
 import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
@@ skipping 21 matching lines @@
 ''';
 
   /**
    * Check that an edit.importElements request with the given list of [elements]
    * produces the [expected] list of edits.
    */
   checkEdits(List<ImportedElements> elements, List<SourceEdit> expected) async {
     EditImportElementsResult result =
         await sendEditImportElements(pathname, elements);
 
-    expect(result.edits, hasLength(expected.length));
+    SourceFileEdit edit = result.edit;
+    expect(edit, isNotNull);
+    expect(edit.edits, hasLength(expected.length));
     // TODO(brianwilkerson) Finish implementing this.
   }
 
   /**
    * Check that an edit.importElements request with the given list of [elements]
    * produces no edits.
    */
   Future<Null> checkNoEdits(List<ImportedElements> elements) async {
     EditImportElementsResult result =
         await sendEditImportElements(pathname, <ImportedElements>[]);
 
-    expect(result.edits, hasLength(0));
+    SourceFileEdit edit = result.edit;
+    expect(edit, isNotNull);
+    expect(edit.edits, hasLength(0));
   }
 
   setUp() {
     return super.setUp().then((_) {
       pathname = sourcePath('test.dart');
     });
   }
 
   test_getImportedElements_noEdits() async {
     writeFile(pathname, text);
     standardAnalysisSetup();
     await analysisFinished;
 
     List<Future> tests = [];
     // Test that an empty list of elements produces no edits.
     tests.add(checkNoEdits(<ImportedElements>[]));
     return Future.wait(tests);
   }
 }