Correctly handle adding imports when pasting into part files

pkg/analysis_server/lib/src/edit/edit_domain.dart

Index: pkg/analysis_server/lib/src/edit/edit_domain.dart
diff --git a/pkg/analysis_server/lib/src/edit/edit_domain.dart b/pkg/analysis_server/lib/src/edit/edit_domain.dart
index f93dd7991d4c79dc26d5392c438a40b172bb2dbf..8007442c2f0fb9a66f150fab3beadf9c116e6129 100644
--- a/pkg/analysis_server/lib/src/edit/edit_domain.dart
+++ b/pkg/analysis_server/lib/src/edit/edit_domain.dart
@@ -397,6 +397,16 @@ class EditDomainHandler extends AbstractRequestHandler {
     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.
     //