FocusTrap: 'Node is not defined' in server side rendering

[leo6104]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

the error not occur.

What is the current behavior?

0|mapia-mo |     ERROR ReferenceError: Node is not defined
0|mapia-mo |     at FocusTrap._getFirstTabbableElement (/var/app/current/server.js:28081:75)
0|mapia-mo |     at FocusTrap._getRegionBoundary (/var/app/current/server.js:27994:55)
0|mapia-mo |     at FocusTrap.focusFirstTabbableElement (/var/app/current/server.js:28038:55)
0|mapia-mo |     at FocusTrap.focusInitialElement (/var/app/current/server.js:28023:21)
0|mapia-mo |     at SafeSubscriber._next (/var/app/current/server.js:27917:71)
0|mapia-mo |     at SafeSubscriber.__tryOrUnsub (/var/app/current/server.js:27392:16)
0|mapia-mo |     at SafeSubscriber.next (/var/app/current/server.js:27339:22)
0|mapia-mo |     at Subscriber._next (/var/app/current/server.js:27279:26)
0|mapia-mo |     at Subscriber.next (/var/app/current/server.js:27243:18)
0|mapia-mo |     at TakeSubscriber._next (/var/app/current/server.js:41499:30)
0|mapia-mo |     at TakeSubscriber.Subscriber.next (/var/app/current/server.js:27243:18)
0|mapia-mo |     at SafeSubscriber.schedulerFn [as _next] (/var/app/current/server.js:4610:52)
0|mapia-mo |     at SafeSubscriber.__tryOrUnsub (/var/app/current/server.js:27392:16)
0|mapia-mo |     at SafeSubscriber.next (/var/app/current/server.js:27339:22)
0|mapia-mo |     at Subscriber._next (/var/app/current/server.js:27279:26)
0|mapia-mo |     at Subscriber.next (/var/app/current/server.js:27243:18)
0|mapia-mo |     at EventEmitter.Subject.next (/var/app/current/server.js:24318:25)
0|mapia-mo |     at EventEmitter.emit (/var/app/current/server.js:4590:46)
0|mapia-mo |     at /var/app/current/server.js:4995:79
0|mapia-mo |     at ZoneDelegate.invoke (/var/app/current/server.js:154462:26)
0|mapia-mo |     at Zone.run (/var/app/current/server.js:154212:43)
0|mapia-mo |     at NgZone.runOutsideAngular (/var/app/current/server.js:4972:69)
0|mapia-mo |     at checkStable (/var/app/current/server.js:4995:26)
0|mapia-mo |     at Object.onHasTask (/var/app/current/server.js:5037:21)
0|mapia-mo |     at ZoneDelegate.hasTask (/var/app/current/server.js:154515:37)
0|mapia-mo |     at ZoneDelegate._updateTaskCount (/var/app/current/server.js:154535:22)
0|mapia-mo |     at Zone._updateTaskCount (/var/app/current/server.js:154359:34)
0|mapia-mo |     at Zone.runTask (/var/app/current/server.js:154279:30)
0|mapia-mo |     at ZoneTask.invokeTask (/var/app/current/server.js:154569:34)
0|mapia-mo |     at ZoneTask.invoke (/var/app/current/server.js:154558:48)
0|mapia-mo |     at data.args.(anonymous function) (/var/app/current/server.js:155251:25)
0|mapia-mo |     at _combinedTickCallback (internal/process/next_tick.js:138:11)
0|mapia-mo |     at process._tickDomainCallback (internal/process/next_tick.js:218:9)

What are the steps to reproduce?

What is the use-case or motivation for changing an existing behavior?

in ssr, sidenav open and focustrap trigger.

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

angular 5.1.1
material2 5.0.1

Is there anything else we should know?

https://github.com/angular/material2/blob/623be22e58d0074a109ac03bdf00afa7c60291ac/src/cdk/a11y/focus-trap.ts#L226
https://github.com/angular/material2/blob/623be22e58d0074a109ac03bdf00afa7c60291ac/src/cdk/a11y/focus-trap.ts#L248
this line might be cause

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