move DartCompletionSorter to dart specific contributor

pkg/analysis_server/lib/src/services/completion/dart/common_usage_sorter.dart

Index: pkg/analysis_server/lib/src/services/completion/dart/common_usage_sorter.dart
diff --git a/pkg/analysis_server/lib/src/services/completion/dart/common_usage_sorter.dart b/pkg/analysis_server/lib/src/services/completion/dart/common_usage_sorter.dart
index 312037eb8b75378509812a6d3931c31bbd9fc90d..6e8049b1c4570b49833b96cca9cff71c4d0af2f4 100644
--- a/pkg/analysis_server/lib/src/services/completion/dart/common_usage_sorter.dart
+++ b/pkg/analysis_server/lib/src/services/completion/dart/common_usage_sorter.dart
@@ -38,8 +38,8 @@ class CommonUsageSorter implements DartContributionSorter {
   CommonUsageSorter([this.selectorRelevance = defaultSelectorRelevance]);
 
   @override
-  Future sort(
-      CompletionRequest request, Iterable<CompletionSuggestion> suggestions) {
+  Future sort(DartCompletionRequest request,
+      Iterable<CompletionSuggestion> suggestions) {
     _update(request, suggestions);
     return new Future.value();
   }