feat: add dart code for chapter_tree (#448)

codes/dart/chapter_tree/binary_tree.dart

@@ -0,0 +1,37 @@
+/**
+ * File: binary_tree.dart
+ * Created Time: 2023-04-03
+ * Author: liuyuxin (gvenusleo@gmail.com)
+ */
+
+import '../utils/print_util.dart';
+import '../utils/tree_node.dart';
+
+void main() {
+  /* 初始化二叉树 */
+  // 舒适化结点
+  TreeNode n1 = TreeNode(1);
+  TreeNode n2 = TreeNode(2);
+  TreeNode n3 = TreeNode(3);
+  TreeNode n4 = TreeNode(4);
+  TreeNode n5 = TreeNode(5);
+  // 构建引用指向（即指针）
+  n1.left = n2;
+  n1.right = n3;
+  n2.left = n4;
+  n2.right = n5;
+  print("\n初始化二叉树\n");
+  printTree(n1);
+
+  /* 插入与删除结点 */
+  TreeNode p = TreeNode(0);
+  // 在 n1 -> n2 中间插入节点 p
+  n1.left = p;
+  p.left = n2;
+  print("\n插入结点 P 后\n");
+  printTree(n1);
+  // 删除结点 P
+  n1.left = n2;
+  print("\n删除结点 P 后\n");
+  printTree(n1);
+}