src/vs/workbench/common/editor/editorGroupModel.ts/EditorGroupModel/contains
class EditorGroupModel:	contains(candidate: EditorInput | IUntypedEditorInput, options?: IMatchEditorOptions): boolean {
		for (const editor of this.editors) {
			if (this.matches(editor, candidate, options)) {
				return true;
			}
		}

		return false;
	}