Added Dart examples for chapter 3 to chapter 9 (#265)

* fix: corrected method return value following Dart's newest linter version

* feat: added Dart recursion examples for chapter 3

* feat: added quicksort example in Dart for chapter 4

* feat: added examples in Dart for the chapter 5

* feat: added Dart example for chapter 6 bfs

* feat: added djikstra example in Dart for chapter 7

* feat: added example of set covering in Dart for chapter 8

* feat: added examples for dynamic programming in dart for chapter 9

06_breadth-first_search/dart/01_breadth-first_search.dart

@@ -0,0 +1,42 @@
+import 'dart:collection';
+
+void main(List<String> args) {
+  final graph = <String, List<String>>{};
+  graph.addAll(
+    <String, List<String>>{
+      'you': ['alice', 'bob', 'claire'],
+      'bob': ['anuj', 'peggy'],
+      'alice': ['peggy'],
+      'claire': ['thom', 'jonny'],
+      'anuj': [],
+      'peggy': [],
+      'thom': [],
+      'jonny': [],
+    },
+  );
+
+  search(graph, 'you');
+}
+
+bool search(Map<String, List<String>> graph, String name) {
+  final searchQueue = Queue()..addAll(graph[name] ?? []);
+  final searched = List<String>.empty(growable: true);
+
+  while (searchQueue.isNotEmpty) {
+    final String person = searchQueue.removeFirst();
+    if (searched.contains(person) == false) {
+      if (_personIsSeller(person)) {
+        print('$person is a Mango seller!');
+        return true;
+      } else {
+        searchQueue.addAll(graph[person] ?? []);
+        searched.add(person);
+      }
+    }
+  }
+  return false;
+}
+
+bool _personIsSeller(String name) {
+  return name.endsWith('m');
+}