fix(checkbox): focus origin for focus method (#3763)

* Currently when developers call the `focus` method on the checkbox, the checkbox focuses via keyboard. This makes the focus-classes of the `FocusOriginMonitor` invalid.
* To be consistent with native input controls, the focus indicator should also show for programmatic focus.

src/lib/checkbox/checkbox.spec.ts

@@ -369,6 +369,23 @@ describe('MdCheckbox', () => {
           .toBe(0, 'Expected no ripple after element is blurred.');
     }));
 
+    it('should show a ripple when focused programmatically', fakeAsync(() => {
+      expect(fixture.nativeElement.querySelectorAll('.mat-ripple-element').length)
+        .toBe(0, 'Expected no ripples to be present.');
+
+      fakeFocusOriginMonitorSubject.next('program');
+      tick(RIPPLE_FADE_IN_DURATION);
+
+      expect(fixture.nativeElement.querySelectorAll('.mat-ripple-element').length)
+        .toBe(1, 'Expected focus ripple to be present.');
+
+      dispatchFakeEvent(checkboxInstance._inputElement.nativeElement, 'blur');
+      tick(RIPPLE_FADE_OUT_DURATION);
+
+      expect(fixture.nativeElement.querySelectorAll('.mat-ripple-element').length)
+        .toBe(0, 'Expected focus ripple to be removed.');
+    }));
+
     describe('ripple elements', () => {
 
       it('should show ripples on label mousedown', () => {

src/lib/checkbox/checkbox.ts

@@ -199,7 +199,7 @@ export class MdCheckbox implements ControlValueAccessor, AfterViewInit, OnDestro
     this._focusedSubscription = this._focusOriginMonitor
       .monitor(this._inputElement.nativeElement, this._renderer, false)
       .subscribe(focusOrigin => {
-        if (!this._focusedRipple && focusOrigin === 'keyboard') {
+        if (!this._focusedRipple && (focusOrigin === 'keyboard' || focusOrigin === 'program')) {
           this._focusedRipple = this._ripple.launch(0, 0, { persistent: true, centered: true });
         }
       });
@@ -392,7 +392,7 @@ export class MdCheckbox implements ControlValueAccessor, AfterViewInit, OnDestro
 
   /** Focuses the checkbox. */
   focus(): void {
-    this._focusOriginMonitor.focusVia(this._inputElement.nativeElement, this._renderer, 'keyboard');
+    this._focusOriginMonitor.focusVia(this._inputElement.nativeElement, this._renderer, 'program');
   }
 
   _onInteractionEvent(event: Event) {