It would be better not to mutate the option disabled state based on t…

…he parent state; this can be error prone, as it's easy to miss updating one when the other changes. It's better to rely on the disabled getter for the option to account for the parent state.

src/lib/list/list-option.ts

@@ -116,7 +116,7 @@ export class MdListOption implements AfterContentInit, OnDestroy, Focusable {
     this._lineSetter = new MdLineSetter(this._lines, this._renderer, this._element);
 
     if (this.selectionList.disabled) {
-      this._disabled = true;
+      this.disabled(true);
     }
   }