fix: don't sort values in non-multiple mode

Author: crisbeto

src/lib/select/select.ts

@@ -579,8 +579,10 @@ export class MdSelect implements AfterContentInit, ControlValueAccessor, OnDestr
    * order that they have in the panel.
    */
   private _sortValues(): void {
-    const options = this.options.toArray();
-    this._selectionModel.sort((a, b) => options.indexOf(a) - options.indexOf(b));
+    if (this._multiple) {
+      const options = this.options.toArray();
+      this._selectionModel.sort((a, b) => options.indexOf(a) - options.indexOf(b));
+    }
   }
 
   /** Unsubscribes from all option subscriptions. */