fix(button): add return type to _getHostElement for type safety

Author: codef0rmerz

src/material/button/button.ts

@@ -118,7 +118,7 @@ export class MatButton extends _MatButtonMixinBase
     this._getHostElement().focus();
   }
 
-  _getHostElement() {
+  _getHostElement(): HTMLElement {
     return this._elementRef.nativeElement;
   }