Allow replacementOffset and Length overriding

pkg/analysis_server/lib/src/domain_completion.dart

Index: pkg/analysis_server/lib/src/domain_completion.dart
diff --git a/pkg/analysis_server/lib/src/domain_completion.dart b/pkg/analysis_server/lib/src/domain_completion.dart
index a00617f22f630dbfa41cec09a5e82fe4a60abbf3..7bd146f599da4bd26c14d33b6f18290881fb0acb 100644
--- a/pkg/analysis_server/lib/src/domain_completion.dart
+++ b/pkg/analysis_server/lib/src/domain_completion.dart
@@ -121,6 +121,16 @@ class CompletionDomainHandler extends AbstractRequestHandler {
       for (plugin.Response response in responses) {
         plugin.CompletionGetSuggestionsResult result =
             new plugin.CompletionGetSuggestionsResult.fromResponse(response);
+        if (suggestions.isEmpty) {
+          request.replacementOffset = result.replacementOffset;
+          request.replacementLength = result.replacementLength;

[Brian Wilkerson, 2017/05/30 18:30:22]

I think we only want to overwrite the current values if there are no current
values. I suspect that you're assuming that if there are no suggestions that
server won't have set these values into the request, but I'm not sure that's
always true. We could make it safe by replacing "=" with "??=".

[maxkim, 2017/05/30 19:10:32]

Adding the null-aware assignment is making the replacementOffset not be properly
override by the plugin's result.replacementOffset; I believe the length is
working fine since the value is null. 

I don't fully understand why the original `request.replacementOffset` and
`request.replacementLength` being null matters if suggestions is already empty.
By this point, the primary Dart autocompletion has had time to finish its
autocompletion calculations and if suggestions is empty, it means there is no
danger of overriding these two values. 

However, I do think I should add another check to make sure that the plugin has
suggestions in the first place before even entering this block. 

[Brian Wilkerson, 2017/05/30 19:18:48]

I'm probably just being too cautious. I'm willing to leave it as is.
That sounds like a good idea.

+        } else if (request.replacementOffset != result.replacementOffset &&
+            request.replacementLength != result.replacementLength) {
+          server.instrumentationService
+              .logError('Plugin completion-results dropped due to conflicting'
+                  ' replacement offset/length: ${result.toJson()}');
+          continue;
+        }
         suggestions.addAll(result.results);
       }
     }