Just faced the same issue,
Did the following and works fine
It occurs mostly because of the iron-router package and few other packages which are using spark
as dependency and first re-install the iron-router like below
meteor remove iron-router
rm -rf packages/iron-router
mrt update
mrt add iron-router
Just re-installing the iron-router will update you to the new version and also it installs blaze-Layout
automatically with the iron-router(which is the new templating system).
After updating iron-router,Even now if you are getting spark
is not defined error,check which package is using spark
and update those packages too.