message: Add back compat to support loader version 0.21.
diff: @@ -552,8 +552,12 @@ implements IContainerRuntime, IContainerRuntimeDirtyable, IRuntime, ISummarizerR
  }
  public get attachState(): AttachState {
  + if (this.context.attachState !== undefined) {
  return this.context.attachState;
  }
  + // 0.21 back-compat isAttached
  + return (this.context as any).isAttached() ? AttachState.Attached : AttachState.Detached;
  + }
  public nextSummarizerP?: Promise<Summarizer>;
  public nextSummarizerD?: Deferred<Summarizer>;
  