getContext() {
    return this.context_;
  }
Get a proxy for CanvasRenderingContext2D which does not support getting state
(e.g. `context.globalAlpha`, which will return `undefined`). To set state, if it relies on a
previous state (e.g. `context.globalAlpha = context.globalAlpha / 2`), set a function,
e.g. `context.globalAlpha = (context) => context.globalAlpha / 2`.
@return {ZIndexContextProxy} Context.