tree.treeControl.expandAll() has an error

#### Bug
Dear team!
I has an error when I want to expand all node of Mat nested tree
```
ERROR TypeError: Cannot read property 'reduce' of undefined
at NestedTreeControl.push../node_modules/@angular/cdk/esm5/tree.es5.js.NestedTreeControl.expandAll (tree.es5.js:287)
at Object.eval [as handleEvent] (FilterViewComponent.html:18)
at handleEvent (core.js:10050)
at callWithDebugContext (core.js:11143)
at Object.debugHandleEvent [as handleEvent] (core.js:10846)
at dispatchEvent (core.js:7509)
at core.js:7953
at HTMLButtonElement. (platform-browser.js:1140)
at ZoneDelegate.push../node_modules/zone.js/dist/zone.js.ZoneDelegate.invokeTask (zone.js:421)
at Object.onInvokeTask (core.js:3748)
```

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Angular 6.1.0
Material 6.4.1

首先使用 FilterViewComponent.html 中所示的调用，在 Angular 6.1.0 和 Material 6.4.1 下重现 NestedTreeControl.expandAll 中的失败。检查 tree.es5.js 中报告位置附近的嵌套树实现，并确定 reduce 为什么会接收到 undefined；当 expandAll 能够无错误地展开所有嵌套节点，并且具有回归测试覆盖时，即表示完成。

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