Correctly handle adding imports when pasting into part files

pkg/analysis_server/lib/src/edit/edit_domain.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/plugin/edit/fix/fix_core.dart';
 import 'package:analysis_server/plugin/edit/fix/fix_dart.dart';
@@ skipping 379 matching lines @@
   Future<Null> importElements(Request request) async {
     EditImportElementsParams params =
         new EditImportElementsParams.fromRequest(request);
     //
     // Prepare the resolved unit.
     //
     AnalysisResult result = await server.getAnalysisResult(params.file);
     if (result == null) {
       server.sendResponse(new Response.importElementsInvalidFile(request));
     }
+    CompilationUnitElement libraryUnit =
+        result.libraryElement.definingCompilationUnit;
+    if (libraryUnit != result.unit.element) {
+      // The file in the request is a part of a library. We need to pass the

[devoncarew, 2017/08/15 21:59:30]

Just so I understand, if you pass a part into `edit.importElements`, you'll get
an error. It's the responsibility of the client to know to go from the part to
the defining library? (and, to gracefully handle errors)

[Brian Wilkerson, 2017/08/15 23:32:33]

No. If the client passes a part into `edit.importElements`, it's the
responsibility of this method to find the defining compilation unit for the
library so that the library unit can be passed in to the
`ImportElementsComputer`. Clients are not expected to have enough Dart-specific
knowledge to find libraries associated with parts.

The client will only get an error if the file passed in to `edit.importElements`
isn't being analyzed or if the file is a part and we can't find the library that
the part belongs to. But, yes, the client needs to gracefully handle errors.

+      // defining compilation unit to the computer, not the part.
+      result = await server.getAnalysisResult(libraryUnit.source.fullName);
+      if (result == null) {
+        server.sendResponse(new Response.importElementsInvalidFile(request));
+      }
+    }
     //
     // Compute the edits required to import the required elements.
     //
     ImportElementsComputer computer =
         new ImportElementsComputer(server.resourceProvider, result);
     SourceChange change = await computer.createEdits(params.elements);
     //
     // Send the response.
     //
     server.sendResponse(
@@ skipping 648 matching lines @@
     }
     return new RefactoringStatus();
   }
 }
 
 /**
  * [_RefactoringManager] throws instances of this class internally to stop
  * processing in a manager that was reset.
  */
 class _ResetError {}