Tree: fixed `default-checked-keys` display wrong (#13349)

* fix: 12383 Tree in lazy mode set child node 'default-checked-keys', display wrong：All nodes are fulfilled

* 添加测试

* fix code style

* update style

packages/tree/src/model/node.js

@@ -465,6 +465,7 @@ export default class Node {
         this.doCreateChildren(children, defaultProps);
 
         this.updateLeafState();
+        reInitChecked(this);
         if (callback) {
           callback.call(this, children);
         }

test/unit/specs/tree.spec.js

@@ -299,6 +299,36 @@ describe('Tree', () => {
     expect(vm.$el.querySelectorAll('.el-checkbox .is-checked').length).to.equal(1);
   });
 
+  it('defaultCheckedKeys & lazy, checked children length as expected', () => {
+    vm = getTreeVm(':load="loadNode" :props="defaultProps" :default-checked-keys="defaultCheckedKeys" node-key="id" :default-expanded-keys="[1]" lazy show-checkbox ', {
+      created() {
+        this.defaultCheckedKeys = [2, 3];
+      },
+      methods: {
+        loadNode(node, resolve) {
+          if (node.level === 0) {
+            return resolve([{ label: 'head', id: 1} ]);
+          }
+          return resolve([
+            {
+              label: '#1',
+              id: 2
+            },
+            {
+              label: '#3',
+              id: 3
+            },
+            {
+              label: '$4',
+              id: 5
+            }
+          ]);
+        }
+      }
+    });
+    expect(vm.$el.querySelectorAll('.el-checkbox.is-checked').length).to.equal(2);
+  });
+
   it('show checkbox', done => {
     vm = getTreeVm(':props="defaultProps" show-checkbox');
     const tree = vm.$children[0];