[rwjblue]

Exposes a few new methods / properties on Transition instances during
debugging (but not in production):

interface Transition {
  /**
    In non-production builds, this function will return the stack that this Transition was
    created within. In production builds, this function will not be present.

    @method debugCreationStack
    @return string
  */
  debugCreationStack?: () => string | undefined;

  /**
    In non-production builds, this function will return the stack that this Transition was
    aborted within (or `undefined` if the Transition has not been aborted yet). In production
    builds, this function will not be present.

    @method debugAbortStack
    @return string
  */
  debugAbortStack?: () => string | undefined;

  /**
    In non-production builds, this property references the Transition that _this_ Transition
    was derived from or `undefined` if this transition did not derive from another. In
    production builds, this property will not be present.

    @property debugPreviousTransition
    @type {Transition | undefined}
  */
  debugPreviousTransition: Maybe<Transition<T>>;
}

From emberjs/router.js#305