fix tests

Author: mmalerba

src/lib/core/datetime/index.ts

@@ -6,10 +6,11 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
+import {PlatformModule} from '@angular/cdk/platform';
 import {NgModule} from '@angular/core';
 import {DateAdapter, MAT_DATE_LOCALE_PROVIDER} from './date-adapter';
-import {NativeDateAdapter} from './native-date-adapter';
 import {MAT_DATE_FORMATS} from './date-formats';
+import {NativeDateAdapter} from './native-date-adapter';
 import {MAT_NATIVE_DATE_FORMATS} from './native-date-formats';
 
 export * from './date-adapter';
@@ -19,6 +20,7 @@ export * from './native-date-formats';
 
 
 @NgModule({
+  imports: [PlatformModule],
   providers: [
     {provide: DateAdapter, useClass: NativeDateAdapter},
     MAT_DATE_LOCALE_PROVIDER