Fix recently introduced strong mode errors

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 b448251c407cfb17ba97b5b45dc964a617e24d2e..719221e8f035bfaaaaa12bb07cc689028338dd64 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
@@ -118,7 +118,7 @@ class CommonUsageSorter implements DartContributionSorter {
 /**
  * An [AstVisitor] used to determine the best defining type of a node.
  */
-class _BestTypeVisitor extends GeneralizingAstVisitor {
+class _BestTypeVisitor extends GeneralizingAstVisitor<DartType> {
   /**
    * The entity which the completed text will replace (or which will be
    * displaced once the completed text is inserted).  This may be an AstNode or