Simplify conversion of empty lists to null.

pkg/analysis_server/lib/src/collections.dart

Index: pkg/analysis_server/lib/src/collections.dart
diff --git a/pkg/analysis_server/lib/src/collections.dart b/pkg/analysis_server/lib/src/collections.dart
index f85bc975cffdbe72a7cf396956b40879ec5cedc6..e49a0fe7f22f00cbd7557f38264eb6eda5c982ba 100644
--- a/pkg/analysis_server/lib/src/collections.dart
+++ b/pkg/analysis_server/lib/src/collections.dart
@@ -16,3 +16,17 @@ Iterable concat(Iterable<Iterable> iterables) => iterables.expand((x) => x);
  * Returns the concatentation of the input iterables as a [List].
  */
 List concatToList(Iterable<Iterable> iterables) => concat(iterables).toList();
+
+
+/**
+ * Returns the given [list] if it is not empty, or `null` otherwise.
+ */
+List nullIfEmpty(List list) {
+  if (list == null) {
+    return null;
+  }
+  if (list.isEmpty) {
+    return null;
+  }
+  return list;
+}