Use HashMap/HashSet for stronglyConnectedComponents (#26)

Looks like ~10% improvement with dart2js and a ~8% improvement on VM

Fixes dart-archive/graphs#25

Author: kevmoo

pkgs/graphs/CHANGELOG.md

@@ -1,3 +1,8 @@
+# 0.1.3
+
+- Use `HashMap` and `HashSet` from `dart:collection` for 
+  `stronglyConnectedComponents`. Improves runtime performance.
+
 # 0.1.2+1
 
 - Allow using non-dev Dart 2 SDK.

pkgs/graphs/lib/src/strongly_connected_components.dart

@@ -26,9 +26,9 @@ import 'dart:math' show min;
 List<List<V>> stronglyConnectedComponents<K, V>(
     Iterable<V> nodes, K Function(V) key, Iterable<V> Function(V) children) {
   final result = <List<V>>[];
-  final lowLinks = <K, int>{};
-  final indexes = <K, int>{};
-  final onStack = Set<K>();
+  final lowLinks = HashMap<K, int>();
+  final indexes = HashMap<K, int>();
+  final onStack = HashSet<K>();
 
   var index = 0;
   var lastVisited = Queue<V>();