fix(slide-toggle) disabled state from a FormControl not updating (#4797)

When setting the disabled state of a slide-toggle using the FormControl's `enable()` and `disable()` methods the slide-toggle's view won't be updated properly.

Fixes #4774

src/lib/slide-toggle/slide-toggle.spec.ts

@@ -618,6 +618,7 @@ describe('MdSlideToggle', () => {
 
     let testComponent: SlideToggleWithFormControl;
     let slideToggle: MdSlideToggle;
+    let inputElement: HTMLInputElement;
 
     beforeEach(() => {
       fixture = TestBed.createComponent(SlideToggleWithFormControl);
@@ -625,20 +626,24 @@ describe('MdSlideToggle', () => {
 
       testComponent = fixture.debugElement.componentInstance;
       slideToggle = fixture.debugElement.query(By.directive(MdSlideToggle)).componentInstance;
+      inputElement = fixture.debugElement.query(By.css('input')).nativeElement;
     });
 
     it('should toggle the disabled state', () => {
       expect(slideToggle.disabled).toBe(false);
+      expect(inputElement.disabled).toBe(false);
 
       testComponent.formControl.disable();
       fixture.detectChanges();
 
       expect(slideToggle.disabled).toBe(true);
+      expect(inputElement.disabled).toBe(true);
 
       testComponent.formControl.enable();
       fixture.detectChanges();
 
       expect(slideToggle.disabled).toBe(false);
+      expect(inputElement.disabled).toBe(false);
     });
   });
 });

src/lib/slide-toggle/slide-toggle.ts

@@ -1,6 +1,7 @@
 import {
   AfterContentInit,
   ChangeDetectionStrategy,
+  ChangeDetectorRef,
   Component,
   ElementRef,
   EventEmitter,
@@ -121,7 +122,8 @@ export class MdSlideToggle extends _MdSlideToggleMixinBase
 
   constructor(private _elementRef: ElementRef,
               private _renderer: Renderer2,
-              private _focusOriginMonitor: FocusOriginMonitor) {
+              private _focusOriginMonitor: FocusOriginMonitor,
+              private _changeDetectorRef: ChangeDetectorRef) {
     super();
   }
 
@@ -190,6 +192,7 @@ export class MdSlideToggle extends _MdSlideToggleMixinBase
   /** Implemented as a part of ControlValueAccessor. */
   setDisabledState(isDisabled: boolean): void {
     this.disabled = isDisabled;
+    this._changeDetectorRef.markForCheck();
   }
 
   /** Focuses the slide-toggle. */