What kind of tree is this (Seattle, WA)?
It looks like a Laburnum. Probably a Common Laburnum Laburnum anagyroides, also known as a Golden Chain Tree. It is from central and southern Europe.

The seeds are poisonous.

https://en.wikipedia.org/wiki/Laburnum_anagyroides

Here is a photo of the leaves

From http://www.saps.org.uk/trees/laburnum.htm

And the flower

http://wildflowerfinder.org.uk/Flowers/L/Laburnum/Laburnum.htm