fix(platform): change isBrowser check to use Angular PLATFORM_ID

* Use the Angular PLATFORM_ID token as a more reliable check for
  whether the current platform is browser. This is because the
  previous check for a global document could be a false positive
  if another application pollutes the global namespace

Author: CaerusKaru

src/cdk/platform/platform.ts

@@ -6,7 +6,8 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {Injectable} from '@angular/core';
+import {Inject, Injectable, PLATFORM_ID} from '@angular/core';
+import {isPlatformBrowser} from '@angular/common';
 
 
 // Whether the current platform supports the V8 Break Iterator. The V8 check
@@ -20,7 +21,7 @@ const hasV8BreakIterator = (typeof Intl !== 'undefined' && (Intl as any).v8Break
 @Injectable({providedIn: 'root'})
 export class Platform {
   /** Whether the Angular application is being rendered in the browser. */
-  isBrowser: boolean = typeof document === 'object' && !!document;
+  isBrowser: boolean = isPlatformBrowser(this._platformId);
 
   /** Whether the current browser is Microsoft Edge. */
   EDGE: boolean = this.isBrowser && /(edge)/i.test(navigator.userAgent);
@@ -37,11 +38,11 @@ export class Platform {
   // Webkit is part of the userAgent in EdgeHTML, Blink and Trident. Therefore we need to
   // ensure that Webkit runs standalone and is not used as another engine's base.
   WEBKIT: boolean = this.isBrowser &&
-      /AppleWebKit/i.test(navigator.userAgent) && !this.BLINK && !this.EDGE && !this.TRIDENT;
+    /AppleWebKit/i.test(navigator.userAgent) && !this.BLINK && !this.EDGE && !this.TRIDENT;
 
   /** Whether the current platform is Apple iOS. */
   IOS: boolean = this.isBrowser && /iPad|iPhone|iPod/.test(navigator.userAgent) &&
-      !(window as any).MSStream;
+    !(window as any).MSStream;
 
   /** Whether the current browser is Firefox. */
   // It's difficult to detect the plain Gecko engine, because most of the browsers identify
@@ -51,12 +52,16 @@ export class Platform {
   FIREFOX: boolean = this.isBrowser && /(firefox|minefield)/i.test(navigator.userAgent);
 
   /** Whether the current platform is Android. */
-  // Trident on mobile adds the android platform to the userAgent to trick detections.
+    // Trident on mobile adds the android platform to the userAgent to trick detections.
   ANDROID: boolean = this.isBrowser && /android/i.test(navigator.userAgent) && !this.TRIDENT;
 
   /** Whether the current browser is Safari. */
   // Safari browsers will include the Safari keyword in their userAgent. Some browsers may fake
   // this and just place the Safari keyword in the userAgent. To be more safe about Safari every
   // Safari browser should also use Webkit as its layout engine.
   SAFARI: boolean = this.isBrowser && /safari/i.test(navigator.userAgent) && this.WEBKIT;
+
+  constructor(@Inject(PLATFORM_ID) private _platformId: Object) {
+  }
 }
+