为什么Angular Dart 2 ViewEncpasulation不再接受枚举？

Question

在Angular Dart 2 Alpha 35中，@ view组件接受了枚举值：

@View(
  styleUrls: const ["package:tickets/client/components/flight_display/flight_display.css"],
  templateUrl: "package:tickets/client/components/flight_display/flight_display.html",
  directives: const[CORE_DIRECTIVES],
  encapsulation: ViewEncapsulation.NONE
)

封装：ViewEncapsulation.NONE现在抛出以下错误：

Error:(11, 18) Arguments of a constant creation must be constant
 expressions

问题： 将值从枚举传递给ViewEncapsulation对象的提供方式是什么？

上下文

以下是Angular.Dart 2 Lib的Enum：

enum ViewEncapsulation {
  /**
   * Emulate scoping of styles by preprocessing the style rules
   * and adding additional attributes to elements. This is the default.
   */
  Emulated,
  /**
   * Uses the native mechanism of the renderer. For the DOM this means creating a ShadowRoot.
   */
  Native,
  /**
   * Don't scope the template nor the styles.
   */
  None
}