No version set in package.json

[RebeccaStevens]

I can't use this repo with yarn as there is no version number in the package.json file.

yarnpkg/yarn#5318

[aomarks]

Polymer 2.x elements are not published on NPM, so they are not installable with npm or yarn. The package.json exists here only for managing devDependencies and npm scripts. Polymer 3.x elements are published on NPM and have a version (more at https://www.polymer-project.org/blog/2018-01-18-polymer-3-new-preview).