chore(a11y): make disabled property optional (#1613)

Author: kara

src/lib/core/a11y/list-key-manager.spec.ts

@@ -77,6 +77,22 @@ describe('ListKeyManager', () => {
     expect(items[2].focus).toHaveBeenCalledTimes(1);
   });
 
+  it('should work normally when disabled property does not exist', () => {
+    items[0].disabled = undefined;
+    items[1].disabled = undefined;
+    items[2].disabled = undefined;
+
+    keyManager.onKeydown(DOWN_ARROW_EVENT);
+    expect(items[0].focus).not.toHaveBeenCalled();
+    expect(items[1].focus).toHaveBeenCalledTimes(1);
+    expect(items[2].focus).not.toHaveBeenCalled();
+
+    keyManager.onKeydown(DOWN_ARROW_EVENT);
+    expect(items[0].focus).not.toHaveBeenCalled();
+    expect(items[1].focus).toHaveBeenCalledTimes(1);
+    expect(items[2].focus).toHaveBeenCalledTimes(1);
+  });
+
   it('should wrap back to menu when arrow keying past items', () => {
     keyManager.onKeydown(DOWN_ARROW_EVENT);
     keyManager.onKeydown(DOWN_ARROW_EVENT);

src/lib/core/a11y/list-key-manager.ts

@@ -7,7 +7,7 @@ import {UP_ARROW, DOWN_ARROW, TAB} from '../core';
  */
 export interface MdFocusable {
   focus(): void;
-  disabled: boolean;
+  disabled?: boolean;
 }
 
 /**