Mat-chip-list multiple selection doesn't work

#### Bug, feature request, or proposal:
Multiple selection of chips doesn't work
Also access to chips.selected is always empty array.

#### What is the expected behavior?
Should be possible to select multiple chips when multiple and selectable are set to true.
Selected array should return array of selected mat-chips.

#### What is the current behavior?
Can select max on chips.
Selected property returns empty array.

#### What are the steps to reproduce?
https://stackblitz.com/edit/angular-5hhmet?file=app/chips-overview-example.ts
check console.log and try to select multiple chips.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Material 5.0.2 and Angular 5.0.5

リンクされたStackBlitzの再現コード、特に app/chips-overview-example.ts から始め、チップを選択したときのコンソール出力を確認します。multiple と selectable が有効な場合の mat-chip-list の動作を追跡します。複数のチップを選択でき、chips.selected が選択された mat-chips の配列を返せば完了です。

索引モデルが issue の本文から書いたものです。