fix(button-toggle): focus monitoring not being cleaned up on destroy

Fixes the focus monitoring for the individual buttons toggles not being cleaned up when they're destroyed, causing the reference to stay inside the `FocusMonitor`.

Author: crisbeto

src/lib/button-toggle/button-toggle.ts

@@ -19,6 +19,7 @@ import {
   EventEmitter,
   forwardRef,
   Input,
+  OnDestroy,
   OnInit,
   Optional,
   Output,
@@ -331,7 +332,8 @@ export const _MatButtonToggleMixinBase = mixinDisableRipple(MatButtonToggleBase)
     '[attr.id]': 'id',
   }
 })
-export class MatButtonToggle extends _MatButtonToggleMixinBase implements OnInit, CanDisableRipple {
+export class MatButtonToggle extends _MatButtonToggleMixinBase implements OnInit,
+  CanDisableRipple, OnDestroy {
 
   private _isSingleSelector = false;
   private _checked = false;
@@ -350,7 +352,7 @@ export class MatButtonToggle extends _MatButtonToggleMixinBase implements OnInit
   /** Type of the button toggle. Either 'radio' or 'checkbox'. */
   _type: ToggleType;
 
-  @ViewChild('input') _inputElement: ElementRef;
+  @ViewChild('input') _inputElement: ElementRef<HTMLInputElement>;
 
   /** The parent button toggle group (exclusive selection). Optional. */
   buttonToggleGroup: MatButtonToggleGroup;
@@ -400,7 +402,7 @@ export class MatButtonToggle extends _MatButtonToggleMixinBase implements OnInit
 
   constructor(@Optional() toggleGroup: MatButtonToggleGroup,
               private _changeDetectorRef: ChangeDetectorRef,
-              private _elementRef: ElementRef,
+              private _elementRef: ElementRef<HTMLElement>,
               private _focusMonitor: FocusMonitor) {
     super();
 
@@ -423,6 +425,10 @@ export class MatButtonToggle extends _MatButtonToggleMixinBase implements OnInit
     this._focusMonitor.monitor(this._elementRef.nativeElement, true);
   }
 
+  ngOnDestroy() {
+    this._focusMonitor.stopMonitoring(this._elementRef.nativeElement);
+  }
+
   /** Focuses the button. */
   focus(): void {
     this._inputElement.nativeElement.focus();