HTMLCollection should be string indexable

According to MDN, HTMLCollection should be string indexable:

HTMLCollection also exposes its members directly as properties by both name and index. HTML IDs may contain : and . as valid characters, which would necessitate using bracket notation for property access. Currently HTMLCollections does not recognize purely numeric IDs, which would cause conflict with the array-style access, though HTML5 does permit these.

TypeScript Version: master

Code

declare const elements: HTMLCollection;
elements['myId'] // implicit any error, where it should be the same type as elements[0].

まず、TypeScript の標準ライブラリ定義にある HTMLCollection の宣言を見つけ、数値プロパティアクセスと名前付きプロパティアクセスを比較します。issue のコード例を使って、文字列によるインデックスアクセスが数値によるインデックスアクセスと同じ要素型を生成することを確認します。名前付きアクセスだけで暗黙の any エラーが報告されなくなれば完了です。