src/vs/workbench/common/editor/editorGroupModel.ts/EditorGroupModel/indexOf
class EditorGroupModel:	indexOf(candidate: IEditorInput | null, editors = this.editors): number {
		if (!candidate) {
			return -1;
		}

		for (let i = 0; i < editors.length; i++) {
			if (this.matches(editors[i], candidate)) {
				return i;
			}
		}

		return -1;
	}