Simplify conversion of empty lists to null.

pkg/analysis_server/lib/src/collections.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library collections;
 
 /**
  * Returns the concatentation of the input iterables.
  *
  * The returned iterable is a lazily-evaluated view on the input iterables.
  */
 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;
+}