Ask Mr. Smarty Plants

  Mr Smarty Plants is going to split this question into two parts, information on selecting plants for a butterfly garden, then how to investigate if those plants are poisonous to dogs.

  The wildflower center has lots of information on Butterfly gardens.  There is a How-to Article, and a Step by Step Guide describing  How to Raise a Butterfly.  To find good plants for your butterfly garden, the Center has lists of Native Plants that are recommended for all the states, and also several specific ecoregions.  This link is to the list for MI, with it already reduced to just the Herbs [of which many are Wildflowers]. 
   Here’s where the fun reading starts! This is as far as the automated searching will take you, but the record that comes up for each wildflower will often state explicitly under “Wildlife” whether they are good for butterflies – and if they are poisonous.   Here are the plants that I found:
Aquilegia canadensis (Eastern red columbine)
Asarum canadense (Canadian wild ginger)  [irritant]
Chamerion angustifolium ssp. angustifolium (Fireweed)
Chelone glabra (White turtlehead)
Coreopsis lanceolata (Lanceleaf coreopsis)
Desmodium canadense (Showy tick trefoil)
Eupatorium perfoliatum (Common boneset)
Fragaria virginiana (Virginia strawberry)
Heuchera richardsonii (Richardson's alumroot)   and this is just the first half of the alphabet

I am listing separately these Milkweeds, they are famous as food sources for the Monarch Butterfly, but unfortunately they are also considered poisionous
Asclepias incarnata (Swamp milkweed),  Asclepias tuberosa (Butterflyweed), Asclepias verticillata (Whorled milkweed)

   On the second question – are the plants toxic to dogs – Mr Smarty Plants has addressed that question several times before.  Rather than just rewriting those question/answer pairs, I have links to several of the better ones here [pay attention to the resources and methods rather than the specific question]:
Evergreen trees for California that are non-toxic for dogs
Is non-native mascagnia macroptera poisonous to animals from Hockley TX
Are the berries of Gun bumelia poisonous to dogs from Willow Park TX
Are magnolia trees toxic for dogs? 
Juniperus Toxic to Pets? 

Unfortunately, the process then is to compare the plants you want to use to the ones on the lists.  If you find one, then you can be reasonably sure that the plant is poisonous as described, but not finding a plant does not guarantee that it is not poisonous. [Too many negatives, but I hope you understand!]  The ASPCA list has the benefit of being specific to dogs.  I used the Canadian list and the one from the University of Illinois which are more generic poisionous rather than specific to dogs because they are more easily searched.  There I found mention [Canadian list] of the Asarium Canadense, but everything else, including the specific species of milkweed, did not show. To my mind - better safe than sorry - and I would exclude these plants [Asarium Canadense and Asclepias Sp.] unless they can be isolated from your dogs.

Happy searching, you have lots of good choices available!