Add hierarchy utils and use them in search.

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 6049add34e0f0a175f41b2e7c7aeb389267c6481..1c5fefc17d7ce90dc79a66af4858d676e7a584c7 100644
--- a/pkg/analysis_server/lib/src/collections.dart
+++ b/pkg/analysis_server/lib/src/collections.dart
@@ -5,6 +5,20 @@
 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.
  */