fix(button): remove disabled attribute when disabled value is false…

… for MdAnchor (#1789)

src/lib/button/button.spec.ts

@@ -127,6 +127,24 @@ describe('MdButton', () => {
       fixture.detectChanges();
       expect(buttonDebugElement.nativeElement.getAttribute('aria-disabled')).toBe('true');
     });
+
+    it('should not add aria-disabled attribute if disabled is false', () => {
+      let fixture = TestBed.createComponent(TestApp);
+      let testComponent = fixture.debugElement.componentInstance;
+      let buttonDebugElement = fixture.debugElement.query(By.css('a'));
+      fixture.detectChanges();
+      expect(buttonDebugElement.nativeElement.getAttribute('aria-disabled'))
+        .toBe('false', 'Expect aria-disabled="false"');
+      expect(buttonDebugElement.nativeElement.getAttribute('disabled'))
+        .toBeNull('Expect disabled="false"');
+
+      testComponent.isDisabled = false;
+      fixture.detectChanges();
+      expect(buttonDebugElement.nativeElement.getAttribute('aria-disabled'))
+        .toBe('false', 'Expect no aria-disabled');
+      expect(buttonDebugElement.nativeElement.getAttribute('disabled'))
+        .toBeNull('Expect no disabled');
+    });
   });
 
   // Ripple tests.

src/lib/button/button.ts

@@ -43,15 +43,15 @@ export class MdButton {
 
   /** Whether the ripple effect on click should be disabled. */
   private _disableRipple: boolean = false;
-  private _disabled: boolean = false;
+  private _disabled: boolean = null;
 
   @Input()
   get disableRipple() { return this._disableRipple; }
   set disableRipple(v) { this._disableRipple = coerceBooleanProperty(v); }
 
   @Input()
   get disabled() { return this._disabled; }
-  set disabled(value: boolean) { this._disabled = coerceBooleanProperty(value); }
+  set disabled(value: boolean) { this._disabled = coerceBooleanProperty(value) ? true : null; }
 
   constructor(private _elementRef: ElementRef, private _renderer: Renderer) { }