fix: handle cloneDeep errors in createLocalVue (#844)

Author: eddyerburgh

Diff for: packages/test-utils/src/create-local-vue.js

@@ -11,8 +11,16 @@ function createLocalVue (_Vue: Component = Vue): Component {
   Object.keys(_Vue).forEach(key => {
     if (!instance.hasOwnProperty(key)) {
       const original = _Vue[key]
-      instance[key] =
-        typeof original === 'object' ? cloneDeep(original) : original
+      // cloneDeep can fail when cloning Vue instances
+      // cloneDeep checks that the instance has a Symbol
+      // which errors in Vue < 2.17 (https://github.com/vuejs/vue/pull/7878)
+      try {
+        instance[key] = typeof original === 'object'
+          ? cloneDeep(original)
+          : original
+      } catch (e) {
+        instance[key] = original
+      }
     }
   })
 

Diff for: test/specs/mounting-options/localVue.spec.js

@@ -83,4 +83,14 @@ describeWithMountingMethods('options.localVue', mountingMethod => {
     })
     expect(localVue.options.created).to.equal(undefined)
   })
+
+  it('handles merging Vue instances', () => {
+    const localVue = createLocalVue()
+    localVue.use((_Vue) => {
+      _Vue.$el = new _Vue()
+    })
+    mountingMethod({ template: '<div />' }, {
+      localVue
+    })
+  })
 })