chore: add PlatformModule to tooltip imports

Author: crisbeto

src/lib/tooltip/tooltip.ts

@@ -32,7 +32,7 @@ import {MdTooltipInvalidPositionError} from './tooltip-errors';
 import {Observable} from 'rxjs/Observable';
 import {Subject} from 'rxjs/Subject';
 import {Dir} from '../core/rtl/dir';
-import {Platform} from '../core/platform/platform';
+import {PlatformModule, Platform} from '../core/platform/index';
 import 'rxjs/add/operator/first';
 
 export type TooltipPosition = 'left' | 'right' | 'above' | 'below' | 'before' | 'after';
@@ -417,7 +417,7 @@ export class TooltipComponent {
 
 
 @NgModule({
-  imports: [OverlayModule, CompatibilityModule],
+  imports: [OverlayModule, CompatibilityModule, PlatformModule],
   exports: [MdTooltip, TooltipComponent, CompatibilityModule],
   declarations: [MdTooltip, TooltipComponent],
   entryComponents: [TooltipComponent],