Add hierarchy utils and use them in search.

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();
+
+
+/**
  * Instances of the class [HasToJson] implement [toJson] method that returns
  * a JSON presentation.
  */
 abstract class HasToJson {
   /**
    * Returns a JSON presentation of the object.
    */
   Map<String, Object> toJson();
 }