Change analysis server protocol to omit empty lists when optional.

pkg/analysis_server/lib/src/computer/computer_occurrences.dart

Index: pkg/analysis_server/lib/src/computer/computer_occurrences.dart
diff --git a/pkg/analysis_server/lib/src/computer/computer_occurrences.dart b/pkg/analysis_server/lib/src/computer/computer_occurrences.dart
index b4f7aa133b78b9057f93279474c5c960ce94531b..124db9344aeeceac60178377985b62aeba19c663 100644
--- a/pkg/analysis_server/lib/src/computer/computer_occurrences.dart
+++ b/pkg/analysis_server/lib/src/computer/computer_occurrences.dart
@@ -31,7 +31,8 @@ class DartUnitOccurrencesComputer {
     _elementsOffsets.forEach((engineElement, offsets) {
       var serverElement = new protocol.Element.fromEngine(engineElement);
       var length = engineElement.displayName.length;
-      occurrences.add(new protocol.Occurrences(serverElement, offsets, length));
+      occurrences.add(new protocol.Occurrences(serverElement, length,
+          offsets: offsets));
     });
     return occurrences;
   }